Frontend/Backend Protocol {#protocol}
=========================

protocol

frontend-backend

PostgreSQL uses a message-based protocol for communication between
frontends and backends (clients and servers). The protocol is supported
over TCP/IP and also over Unix-domain sockets. Port number 5432 has been
registered with IANA as the customary TCP port number for servers
supporting this protocol, but in practice any non-privileged port number
can be used.

This document describes version 3.0 of the protocol, implemented in
PostgreSQL 7.4 and later. For descriptions of the earlier protocol
versions, see previous releases of the PostgreSQL documentation. A
single server can support multiple protocol versions. The initial
startup-request message tells the server which protocol version the
client is attempting to use. If the major version requested by the
client is not supported by the server, the connection will be rejected
(for example, this would occur if the client requested protocol version
4.0, which does not exist as of this writing). If the minor version
requested by the client is not supported by the server (e.g., the client
requests version 3.1, but the server supports only 3.0), the server may
either reject the connection or may respond with a
NegotiateProtocolVersion message containing the highest minor protocol
version which it supports. The client may then choose either to continue
with the connection using the specified protocol version or to abort the
connection.

In order to serve multiple clients efficiently, the server launches a
new "backend" process for each client. In the current implementation, a
new child process is created immediately after an incoming connection is
detected. This is transparent to the protocol, however. For purposes of
the protocol, the terms "backend" and "server" are interchangeable;
likewise "frontend" and "client" are interchangeable.

Overview {#protocol-overview}
========

The protocol has separate phases for startup and normal operation. In
the startup phase, the frontend opens a connection to the server and
authenticates itself to the satisfaction of the server. (This might
involve a single message, or multiple messages depending on the
authentication method being used.) If all goes well, the server then
sends status information to the frontend, and finally enters normal
operation. Except for the initial startup-request message, this part of
the protocol is driven by the server.

During normal operation, the frontend sends queries and other commands
to the backend, and the backend sends back query results and other
responses. There are a few cases (such as `NOTIFY`) wherein the backend
will send unsolicited messages, but for the most part this portion of a
session is driven by frontend requests.

Termination of the session is normally by frontend choice, but can be
forced by the backend in certain cases. In any case, when the backend
closes the connection, it will roll back any open (incomplete)
transaction before exiting.

Within normal operation, SQL commands can be executed through either of
two sub-protocols. In the "simple query" protocol, the frontend just
sends a textual query string, which is parsed and immediately executed
by the backend. In the "extended query" protocol, processing of queries
is separated into multiple steps: parsing, binding of parameter values,
and execution. This offers flexibility and performance benefits, at the
cost of extra complexity.

Normal operation has additional sub-protocols for special operations
such as `COPY`.

Messaging Overview {#protocol-message-concepts}
------------------

All communication is through a stream of messages. The first byte of a
message identifies the message type, and the next four bytes specify the
length of the rest of the message (this length count includes itself,
but not the message-type byte). The remaining contents of the message
are determined by the message type. For historical reasons, the very
first message sent by the client (the startup message) has no initial
message-type byte.

To avoid losing synchronization with the message stream, both servers
and clients typically read an entire message into a buffer (using the
byte count) before attempting to process its contents. This allows easy
recovery if an error is detected while processing the contents. In
extreme situations (such as not having enough memory to buffer the
message), the receiver can use the byte count to determine how much
input to skip before it resumes reading messages.

Conversely, both servers and clients must take care never to send an
incomplete message. This is commonly done by marshaling the entire
message in a buffer before beginning to send it. If a communications
failure occurs partway through sending or receiving a message, the only
sensible response is to abandon the connection, since there is little
hope of recovering message-boundary synchronization.

Extended Query Overview {#protocol-query-concepts}
-----------------------

In the extended-query protocol, execution of SQL commands is divided
into multiple steps. The state retained between steps is represented by
two types of objects: prepared statements and portals. A prepared
statement represents the result of parsing and semantic analysis of a
textual query string. A prepared statement is not in itself ready to
execute, because it might lack specific values for parameters. A portal
represents a ready-to-execute or already-partially-executed statement,
with any missing parameter values filled in. (For `SELECT` statements, a
portal is equivalent to an open cursor, but we choose to use a different
term since cursors don\'t handle non-`SELECT` statements.)

The overall execution cycle consists of a parse step, which creates a
prepared statement from a textual query string; a bind step, which
creates a portal given a prepared statement and values for any needed
parameters; and an execute step that runs a portal\'s query. In the case
of a query that returns rows (`SELECT`, `SHOW`, etc), the execute step
can be told to fetch only a limited number of rows, so that multiple
execute steps might be needed to complete the operation.

The backend can keep track of multiple prepared statements and portals
(but note that these exist only within a session, and are never shared
across sessions). Existing prepared statements and portals are
referenced by names assigned when they were created. In addition, an
"unnamed" prepared statement and portal exist. Although these behave
largely the same as named objects, operations on them are optimized for
the case of executing a query only once and then discarding it, whereas
operations on named objects are optimized on the expectation of multiple
uses.

Formats and Format Codes {#protocol-format-codes}
------------------------

Data of a particular data type might be transmitted in any of several
different formats. As of PostgreSQL 7.4 the only supported formats are
"text" and "binary", but the protocol makes provision for future
extensions. The desired format for any value is specified by a format
code. Clients can specify a format code for each transmitted parameter
value and for each column of a query result. Text has format code zero,
binary has format code one, and all other format codes are reserved for
future definition.

The text representation of values is whatever strings are produced and
accepted by the input/output conversion functions for the particular
data type. In the transmitted representation, there is no trailing null
character; the frontend must add one to received values if it wants to
process them as C strings. (The text format does not allow embedded
nulls, by the way.)

Binary representations for integers use network byte order (most
significant byte first). For other data types consult the documentation
or source code to learn about the binary representation. Keep in mind
that binary representations for complex data types might change across
server versions; the text format is usually the more portable choice.

Message Flow {#protocol-flow}
============

This section describes the message flow and the semantics of each
message type. (Details of the exact representation of each message
appear in [Message Formats](#protocol-message-formats).) There are
several different sub-protocols depending on the state of the
connection: start-up, query, function call, `COPY`, and termination.
There are also special provisions for asynchronous operations (including
notification responses and command cancellation), which can occur at any
time after the start-up phase.

Start-up
--------

To begin a session, a frontend opens a connection to the server and
sends a startup message. This message includes the names of the user and
of the database the user wants to connect to; it also identifies the
particular protocol version to be used. (Optionally, the startup message
can include additional settings for run-time parameters.) The server
then uses this information and the contents of its configuration files
(such as `pg_hba.conf`) to determine whether the connection is
provisionally acceptable, and what additional authentication is required
(if any).

The server then sends an appropriate authentication request message, to
which the frontend must reply with an appropriate authentication
response message (such as a password). For all authentication methods
except GSSAPI, SSPI and SASL, there is at most one request and one
response. In some methods, no response at all is needed from the
frontend, and so no authentication request occurs. For GSSAPI, SSPI and
SASL, multiple exchanges of packets may be needed to complete the
authentication.

The authentication cycle ends with the server either rejecting the
connection attempt (ErrorResponse), or sending AuthenticationOk.

The possible messages from the server in this phase are:

ErrorResponse

:   The connection attempt has been rejected. The server then
    immediately closes the connection.

AuthenticationOk

:   The authentication exchange is successfully completed.

AuthenticationKerberosV5

:   The frontend must now take part in a Kerberos V5 authentication
    dialog (not described here, part of the Kerberos specification) with
    the server. If this is successful, the server responds with an
    AuthenticationOk, otherwise it responds with an ErrorResponse. This
    is no longer supported.

AuthenticationCleartextPassword

:   The frontend must now send a PasswordMessage containing the password
    in clear-text form. If this is the correct password, the server
    responds with an AuthenticationOk, otherwise it responds with an
    ErrorResponse.

AuthenticationMD5Password

:   The frontend must now send a PasswordMessage containing the password
    (with user name) encrypted via MD5, then encrypted again using the
    4-byte random salt specified in the AuthenticationMD5Password
    message. If this is the correct password, the server responds with
    an AuthenticationOk, otherwise it responds with an ErrorResponse.
    The actual PasswordMessage can be computed in SQL as `concat('md5',
            md5(concat(md5(concat(password, username)), random-salt)))`.
    (Keep in mind the `md5()` function returns its result as a hex
    string.)

AuthenticationSCMCredential

:   This response is only possible for local Unix-domain connections on
    platforms that support SCM credential messages. The frontend must
    issue an SCM credential message and then send a single data byte.
    (The contents of the data byte are uninteresting; it\'s only used to
    ensure that the server waits long enough to receive the credential
    message.) If the credential is acceptable, the server responds with
    an AuthenticationOk, otherwise it responds with an ErrorResponse.
    (This message type is only issued by pre-9.1 servers. It may
    eventually be removed from the protocol specification.)

AuthenticationGSS

:   The frontend must now initiate a GSSAPI negotiation. The frontend
    will send a GSSResponse message with the first part of the GSSAPI
    data stream in response to this. If further messages are needed, the
    server will respond with AuthenticationGSSContinue.

AuthenticationSSPI

:   The frontend must now initiate an SSPI negotiation. The frontend
    will send a GSSResponse with the first part of the SSPI data stream
    in response to this. If further messages are needed, the server will
    respond with AuthenticationGSSContinue.

AuthenticationGSSContinue

:   This message contains the response data from the previous step of
    GSSAPI or SSPI negotiation (AuthenticationGSS, AuthenticationSSPI or
    a previous AuthenticationGSSContinue). If the GSSAPI or SSPI data in
    this message indicates more data is needed to complete the
    authentication, the frontend must send that data as another
    GSSResponse message. If GSSAPI or SSPI authentication is completed
    by this message, the server will next send AuthenticationOk to
    indicate successful authentication or ErrorResponse to indicate
    failure.

AuthenticationSASL

:   The frontend must now initiate a SASL negotiation, using one of the
    SASL mechanisms listed in the message. The frontend will send a
    SASLInitialResponse with the name of the selected mechanism, and the
    first part of the SASL data stream in response to this. If further
    messages are needed, the server will respond with
    AuthenticationSASLContinue. See [SASL
    Authentication](#sasl-authentication) for details.

AuthenticationSASLContinue

:   This message contains challenge data from the previous step of SASL
    negotiation (AuthenticationSASL, or a previous
    AuthenticationSASLContinue). The frontend must respond with a
    SASLResponse message.

AuthenticationSASLFinal

:   SASL authentication has completed with additional mechanism-specific
    data for the client. The server will next send AuthenticationOk to
    indicate successful authentication, or an ErrorResponse to indicate
    failure. This message is sent only if the SASL mechanism specifies
    additional data to be sent from server to client at completion.

NegotiateProtocolVersion

:   The server does not support the minor protocol version requested by
    the client, but does support an earlier version of the protocol;
    this message indicates the highest supported minor version. This
    message will also be sent if the client requested unsupported
    protocol options (i.e., beginning with `_pq_.`) in the startup
    packet. This message will be followed by an ErrorResponse or a
    message indicating the success or failure of authentication.

If the frontend does not support the authentication method requested by
the server, then it should immediately close the connection.

After having received AuthenticationOk, the frontend must wait for
further messages from the server. In this phase a backend process is
being started, and the frontend is just an interested bystander. It is
still possible for the startup attempt to fail (ErrorResponse) or the
server to decline support for the requested minor protocol version
(NegotiateProtocolVersion), but in the normal case the backend will send
some ParameterStatus messages, BackendKeyData, and finally
ReadyForQuery.

During this phase the backend will attempt to apply any additional
run-time parameter settings that were given in the startup message. If
successful, these values become session defaults. An error causes
ErrorResponse and exit.

The possible messages from the backend in this phase are:

BackendKeyData

:   This message provides secret-key data that the frontend must save if
    it wants to be able to issue cancel requests later. The frontend
    should not respond to this message, but should continue listening
    for a ReadyForQuery message.

ParameterStatus

:   This message informs the frontend about the current (initial)
    setting of backend parameters, such as [???](#guc-client-encoding)
    or [???](#guc-datestyle). The frontend can ignore this message, or
    record the settings for its future use; see [Asynchronous
    Operations](#protocol-async) for more details. The frontend should
    not respond to this message, but should continue listening for a
    ReadyForQuery message.

ReadyForQuery

:   Start-up is completed. The frontend can now issue commands.

ErrorResponse

:   Start-up failed. The connection is closed after sending this
    message.

NoticeResponse

:   A warning message has been issued. The frontend should display the
    message but continue listening for ReadyForQuery or ErrorResponse.

The ReadyForQuery message is the same one that the backend will issue
after each command cycle. Depending on the coding needs of the frontend,
it is reasonable to consider ReadyForQuery as starting a command cycle,
or to consider ReadyForQuery as ending the start-up phase and each
subsequent command cycle.

Simple Query
------------

A simple query cycle is initiated by the frontend sending a Query
message to the backend. The message includes an SQL command (or
commands) expressed as a text string. The backend then sends one or more
response messages depending on the contents of the query command string,
and finally a ReadyForQuery response message. ReadyForQuery informs the
frontend that it can safely send a new command. (It is not actually
necessary for the frontend to wait for ReadyForQuery before issuing
another command, but the frontend must then take responsibility for
figuring out what happens if the earlier command fails and
already-issued later commands succeed.)

The possible response messages from the backend are:

CommandComplete

:   An SQL command completed normally.

CopyInResponse

:   The backend is ready to copy data from the frontend to a table; see
    [COPY Operations](#protocol-copy).

CopyOutResponse

:   The backend is ready to copy data from a table to the frontend; see
    [COPY Operations](#protocol-copy).

RowDescription

:   Indicates that rows are about to be returned in response to a
    `SELECT`, `FETCH`, etc query. The contents of this message describe
    the column layout of the rows. This will be followed by a DataRow
    message for each row being returned to the frontend.

DataRow

:   One of the set of rows returned by a `SELECT`, `FETCH`, etc query.

EmptyQueryResponse

:   An empty query string was recognized.

ErrorResponse

:   An error has occurred.

ReadyForQuery

:   Processing of the query string is complete. A separate message is
    sent to indicate this because the query string might contain
    multiple SQL commands. (CommandComplete marks the end of processing
    one SQL command, not the whole string.) ReadyForQuery will always be
    sent, whether processing terminates successfully or with an error.

NoticeResponse

:   A warning message has been issued in relation to the query. Notices
    are in addition to other responses, i.e., the backend will continue
    processing the command.

The response to a `SELECT` query (or other queries that return row sets,
such as `EXPLAIN` or `SHOW`) normally consists of RowDescription, zero
or more DataRow messages, and then CommandComplete. `COPY` to or from
the frontend invokes special protocol as described in [COPY
Operations](#protocol-copy). All other query types normally produce only
a CommandComplete message.

Since a query string could contain several queries (separated by
semicolons), there might be several such response sequences before the
backend finishes processing the query string. ReadyForQuery is issued
when the entire string has been processed and the backend is ready to
accept a new query string.

If a completely empty (no contents other than whitespace) query string
is received, the response is EmptyQueryResponse followed by
ReadyForQuery.

In the event of an error, ErrorResponse is issued followed by
ReadyForQuery. All further processing of the query string is aborted by
ErrorResponse (even if more queries remained in it). Note that this
might occur partway through the sequence of messages generated by an
individual query.

In simple Query mode, the format of retrieved values is always text,
except when the given command is a `FETCH` from a cursor declared with
the `BINARY` option. In that case, the retrieved values are in binary
format. The format codes given in the RowDescription message tell which
format is being used.

A frontend must be prepared to accept ErrorResponse and NoticeResponse
messages whenever it is expecting any other type of message. See also
[Asynchronous Operations](#protocol-async) concerning messages that the
backend might generate due to outside events.

Recommended practice is to code frontends in a state-machine style that
will accept any message type at any time that it could make sense,
rather than wiring in assumptions about the exact sequence of messages.

### Multiple Statements in a Simple Query {#protocol-flow-multi-statement}

When a simple Query message contains more than one SQL statement
(separated by semicolons), those statements are executed as a single
transaction, unless explicit transaction control commands are included
to force a different behavior. For example, if the message contains

    INSERT INTO mytable VALUES(1);
    SELECT 1/0;
    INSERT INTO mytable VALUES(2);

then the divide-by-zero failure in the `SELECT` will force rollback of
the first `INSERT`. Furthermore, because execution of the message is
abandoned at the first error, the second `INSERT` is never attempted at
all.

If instead the message contains

    BEGIN;
    INSERT INTO mytable VALUES(1);
    COMMIT;
    INSERT INTO mytable VALUES(2);
    SELECT 1/0;

then the first `INSERT` is committed by the explicit `COMMIT` command.
The second `INSERT` and the `SELECT` are still treated as a single
transaction, so that the divide-by-zero failure will roll back the
second `INSERT`, but not the first one.

This behavior is implemented by running the statements in a
multi-statement Query message in an implicit transaction block unless
there is some explicit transaction block for them to run in. The main
difference between an implicit transaction block and a regular one is
that an implicit block is closed automatically at the end of the Query
message, either by an implicit commit if there was no error, or an
implicit rollback if there was an error. This is similar to the implicit
commit or rollback that happens for a statement executed by itself (when
not in a transaction block).

If the session is already in a transaction block, as a result of a
`BEGIN` in some previous message, then the Query message simply
continues that transaction block, whether the message contains one
statement or several. However, if the Query message contains a `COMMIT`
or `ROLLBACK` closing the existing transaction block, then any following
statements are executed in an implicit transaction block. Conversely, if
a `BEGIN` appears in a multi-statement Query message, then it starts a
regular transaction block that will only be terminated by an explicit
`COMMIT` or `ROLLBACK`, whether that appears in this Query message or a
later one. If the `BEGIN` follows some statements that were executed as
an implicit transaction block, those statements are not immediately
committed; in effect, they are retroactively included into the new
regular transaction block.

A `COMMIT` or `ROLLBACK` appearing in an implicit transaction block is
executed as normal, closing the implicit block; however, a warning will
be issued since a `COMMIT` or `ROLLBACK` without a previous `BEGIN`
might represent a mistake. If more statements follow, a new implicit
transaction block will be started for them.

Savepoints are not allowed in an implicit transaction block, since they
would conflict with the behavior of automatically closing the block upon
any error.

Remember that, regardless of any transaction control commands that may
be present, execution of the Query message stops at the first error.
Thus for example given

    BEGIN;
    SELECT 1/0;
    ROLLBACK;

in a single Query message, the session will be left inside a failed
regular transaction block, since the `ROLLBACK` is not reached after the
divide-by-zero error. Another `ROLLBACK` will be needed to restore the
session to a usable state.

Another behavior of note is that initial lexical and syntactic analysis
is done on the entire query string before any of it is executed. Thus
simple errors (such as a misspelled keyword) in later statements can
prevent execution of any of the statements. This is normally invisible
to users since the statements would all roll back anyway when done as an
implicit transaction block. However, it can be visible when attempting
to do multiple transactions within a multi-statement Query. For
instance, if a typo turned our previous example into

    BEGIN;
    INSERT INTO mytable VALUES(1);
    COMMIT;
    INSERT INTO mytable VALUES(2);
    SELCT 1/0;

then none of the statements would get run, resulting in the visible
difference that the first `INSERT` is not committed. Errors detected at
semantic analysis or later, such as a misspelled table or column name,
do not have this effect.

Extended Query {#protocol-flow-ext-query}
--------------

The extended query protocol breaks down the above-described simple query
protocol into multiple steps. The results of preparatory steps can be
re-used multiple times for improved efficiency. Furthermore, additional
features are available, such as the possibility of supplying data values
as separate parameters instead of having to insert them directly into a
query string.

In the extended protocol, the frontend first sends a Parse message,
which contains a textual query string, optionally some information about
data types of parameter placeholders, and the name of a destination
prepared-statement object (an empty string selects the unnamed prepared
statement). The response is either ParseComplete or ErrorResponse.
Parameter data types can be specified by OID; if not given, the parser
attempts to infer the data types in the same way as it would do for
untyped literal string constants.

> **Note**
>
> A parameter data type can be left unspecified by setting it to zero,
> or by making the array of parameter type OIDs shorter than the number
> of parameter symbols (`$`n) used in the query string. Another special
> case is that a parameter\'s type can be specified as `void` (that is,
> the OID of the `void` pseudo-type). This is meant to allow parameter
> symbols to be used for function parameters that are actually OUT
> parameters. Ordinarily there is no context in which a `void` parameter
> could be used, but if such a parameter symbol appears in a function\'s
> parameter list, it is effectively ignored. For example, a function
> call such as `foo($1,$2,$3,$4)` could match a function with two IN and
> two OUT arguments, if `$3` and `$4` are specified as having type
> `void`.

> **Note**
>
> The query string contained in a Parse message cannot include more than
> one SQL statement; else a syntax error is reported. This restriction
> does not exist in the simple-query protocol, but it does exist in the
> extended protocol, because allowing prepared statements or portals to
> contain multiple commands would complicate the protocol unduly.

If successfully created, a named prepared-statement object lasts till
the end of the current session, unless explicitly destroyed. An unnamed
prepared statement lasts only until the next Parse statement specifying
the unnamed statement as destination is issued. (Note that a simple
Query message also destroys the unnamed statement.) Named prepared
statements must be explicitly closed before they can be redefined by
another Parse message, but this is not required for the unnamed
statement. Named prepared statements can also be created and accessed at
the SQL command level, using `PREPARE` and `EXECUTE`.

Once a prepared statement exists, it can be readied for execution using
a Bind message. The Bind message gives the name of the source prepared
statement (empty string denotes the unnamed prepared statement), the
name of the destination portal (empty string denotes the unnamed
portal), and the values to use for any parameter placeholders present in
the prepared statement. The supplied parameter set must match those
needed by the prepared statement. (If you declared any `void` parameters
in the Parse message, pass NULL values for them in the Bind message.)
Bind also specifies the format to use for any data returned by the
query; the format can be specified overall, or per-column. The response
is either BindComplete or ErrorResponse.

> **Note**
>
> The choice between text and binary output is determined by the format
> codes given in Bind, regardless of the SQL command involved. The
> `BINARY` attribute in cursor declarations is irrelevant when using
> extended query protocol.

Query planning typically occurs when the Bind message is processed. If
the prepared statement has no parameters, or is executed repeatedly, the
server might save the created plan and re-use it during subsequent Bind
messages for the same prepared statement. However, it will do so only if
it finds that a generic plan can be created that is not much less
efficient than a plan that depends on the specific parameter values
supplied. This happens transparently so far as the protocol is
concerned.

If successfully created, a named portal object lasts till the end of the
current transaction, unless explicitly destroyed. An unnamed portal is
destroyed at the end of the transaction, or as soon as the next Bind
statement specifying the unnamed portal as destination is issued. (Note
that a simple Query message also destroys the unnamed portal.) Named
portals must be explicitly closed before they can be redefined by
another Bind message, but this is not required for the unnamed portal.
Named portals can also be created and accessed at the SQL command level,
using `DECLARE CURSOR` and `FETCH`.

Once a portal exists, it can be executed using an Execute message. The
Execute message specifies the portal name (empty string denotes the
unnamed portal) and a maximum result-row count (zero meaning "fetch all
rows"). The result-row count is only meaningful for portals containing
commands that return row sets; in other cases the command is always
executed to completion, and the row count is ignored. The possible
responses to Execute are the same as those described above for queries
issued via simple query protocol, except that Execute doesn\'t cause
ReadyForQuery or RowDescription to be issued.

If Execute terminates before completing the execution of a portal (due
to reaching a nonzero result-row count), it will send a PortalSuspended
message; the appearance of this message tells the frontend that another
Execute should be issued against the same portal to complete the
operation. The CommandComplete message indicating completion of the
source SQL command is not sent until the portal\'s execution is
completed. Therefore, an Execute phase is always terminated by the
appearance of exactly one of these messages: CommandComplete,
EmptyQueryResponse (if the portal was created from an empty query
string), ErrorResponse, or PortalSuspended.

At completion of each series of extended-query messages, the frontend
should issue a Sync message. This parameterless message causes the
backend to close the current transaction if it\'s not inside a
`BEGIN`/`COMMIT` transaction block ("close" meaning to commit if no
error, or roll back if error). Then a ReadyForQuery response is issued.
The purpose of Sync is to provide a resynchronization point for error
recovery. When an error is detected while processing any extended-query
message, the backend issues ErrorResponse, then reads and discards
messages until a Sync is reached, then issues ReadyForQuery and returns
to normal message processing. (But note that no skipping occurs if an
error is detected *while* processing Sync MDASH this ensures that there
is one and only one ReadyForQuery sent for each Sync.)

> **Note**
>
> Sync does not cause a transaction block opened with `BEGIN` to be
> closed. It is possible to detect this situation since the
> ReadyForQuery message includes transaction status information.

In addition to these fundamental, required operations, there are several
optional operations that can be used with extended-query protocol.

The Describe message (portal variant) specifies the name of an existing
portal (or an empty string for the unnamed portal). The response is a
RowDescription message describing the rows that will be returned by
executing the portal; or a NoData message if the portal does not contain
a query that will return rows; or ErrorResponse if there is no such
portal.

The Describe message (statement variant) specifies the name of an
existing prepared statement (or an empty string for the unnamed prepared
statement). The response is a ParameterDescription message describing
the parameters needed by the statement, followed by a RowDescription
message describing the rows that will be returned when the statement is
eventually executed (or a NoData message if the statement will not
return rows). ErrorResponse is issued if there is no such prepared
statement. Note that since Bind has not yet been issued, the formats to
be used for returned columns are not yet known to the backend; the
format code fields in the RowDescription message will be zeroes in this
case.

> **Tip**
>
> In most scenarios the frontend should issue one or the other variant
> of Describe before issuing Execute, to ensure that it knows how to
> interpret the results it will get back.

The Close message closes an existing prepared statement or portal and
releases resources. It is not an error to issue Close against a
nonexistent statement or portal name. The response is normally
CloseComplete, but could be ErrorResponse if some difficulty is
encountered while releasing resources. Note that closing a prepared
statement implicitly closes any open portals that were constructed from
that statement.

The Flush message does not cause any specific output to be generated,
but forces the backend to deliver any data pending in its output
buffers. A Flush must be sent after any extended-query command except
Sync, if the frontend wishes to examine the results of that command
before issuing more commands. Without Flush, messages returned by the
backend will be combined into the minimum possible number of packets to
minimize network overhead.

> **Note**
>
> The simple Query message is approximately equivalent to the series
> Parse, Bind, portal Describe, Execute, Close, Sync, using the unnamed
> prepared statement and portal objects and no parameters. One
> difference is that it will accept multiple SQL statements in the query
> string, automatically performing the bind/describe/execute sequence
> for each one in succession. Another difference is that it will not
> return ParseComplete, BindComplete, CloseComplete, or NoData messages.

Function Call
-------------

The Function Call sub-protocol allows the client to request a direct
call of any function that exists in the database\'s pg\_proc system
catalog. The client must have execute permission for the function.

> **Note**
>
> The Function Call sub-protocol is a legacy feature that is probably
> best avoided in new code. Similar results can be accomplished by
> setting up a prepared statement that does `SELECT function($1, ...)`.
> The Function Call cycle can then be replaced with Bind/Execute.

A Function Call cycle is initiated by the frontend sending a
FunctionCall message to the backend. The backend then sends one or more
response messages depending on the results of the function call, and
finally a ReadyForQuery response message. ReadyForQuery informs the
frontend that it can safely send a new query or function call.

The possible response messages from the backend are:

ErrorResponse

:   An error has occurred.

FunctionCallResponse

:   The function call was completed and returned the result given in the
    message. (Note that the Function Call protocol can only handle a
    single scalar result, not a row type or set of results.)

ReadyForQuery

:   Processing of the function call is complete. ReadyForQuery will
    always be sent, whether processing terminates successfully or with
    an error.

NoticeResponse

:   A warning message has been issued in relation to the function call.
    Notices are in addition to other responses, i.e., the backend will
    continue processing the command.

COPY Operations {#protocol-copy}
---------------

The `COPY` command allows high-speed bulk data transfer to or from the
server. Copy-in and copy-out operations each switch the connection into
a distinct sub-protocol, which lasts until the operation is completed.

Copy-in mode (data transfer to the server) is initiated when the backend
executes a `COPY FROM STDIN` SQL statement. The backend sends a
CopyInResponse message to the frontend. The frontend should then send
zero or more CopyData messages, forming a stream of input data. (The
message boundaries are not required to have anything to do with row
boundaries, although that is often a reasonable choice.) The frontend
can terminate the copy-in mode by sending either a CopyDone message
(allowing successful termination) or a CopyFail message (which will
cause the `COPY` SQL statement to fail with an error). The backend then
reverts to the command-processing mode it was in before the `COPY`
started, which will be either simple or extended query protocol. It will
next send either CommandComplete (if successful) or ErrorResponse (if
not).

In the event of a backend-detected error during copy-in mode (including
receipt of a CopyFail message), the backend will issue an ErrorResponse
message. If the `COPY` command was issued via an extended-query message,
the backend will now discard frontend messages until a Sync message is
received, then it will issue ReadyForQuery and return to normal
processing. If the `COPY` command was issued in a simple Query message,
the rest of that message is discarded and ReadyForQuery is issued. In
either case, any subsequent CopyData, CopyDone, or CopyFail messages
issued by the frontend will simply be dropped.

The backend will ignore Flush and Sync messages received during copy-in
mode. Receipt of any other non-copy message type constitutes an error
that will abort the copy-in state as described above. (The exception for
Flush and Sync is for the convenience of client libraries that always
send Flush or Sync after an Execute message, without checking whether
the command to be executed is a `COPY FROM STDIN`.)

Copy-out mode (data transfer from the server) is initiated when the
backend executes a `COPY TO STDOUT` SQL statement. The backend sends a
CopyOutResponse message to the frontend, followed by zero or more
CopyData messages (always one per row), followed by CopyDone. The
backend then reverts to the command-processing mode it was in before the
`COPY` started, and sends CommandComplete. The frontend cannot abort the
transfer (except by closing the connection or issuing a Cancel request),
but it can discard unwanted CopyData and CopyDone messages.

In the event of a backend-detected error during copy-out mode, the
backend will issue an ErrorResponse message and revert to normal
processing. The frontend should treat receipt of ErrorResponse as
terminating the copy-out mode.

It is possible for NoticeResponse and ParameterStatus messages to be
interspersed between CopyData messages; frontends must handle these
cases, and should be prepared for other asynchronous message types as
well (see [Asynchronous Operations](#protocol-async)). Otherwise, any
message type other than CopyData or CopyDone may be treated as
terminating copy-out mode.

There is another Copy-related mode called copy-both, which allows
high-speed bulk data transfer to *and* from the server. Copy-both mode
is initiated when a backend in walsender mode executes a
`START_REPLICATION` statement. The backend sends a CopyBothResponse
message to the frontend. Both the backend and the frontend may then send
CopyData messages until either end sends a CopyDone message. After the
client sends a CopyDone message, the connection goes from copy-both mode
to copy-out mode, and the client may not send any more CopyData
messages. Similarly, when the server sends a CopyDone message, the
connection goes into copy-in mode, and the server may not send any more
CopyData messages. After both sides have sent a CopyDone message, the
copy mode is terminated, and the backend reverts to the
command-processing mode. In the event of a backend-detected error during
copy-both mode, the backend will issue an ErrorResponse message, discard
frontend messages until a Sync message is received, and then issue
ReadyForQuery and return to normal processing. The frontend should treat
receipt of ErrorResponse as terminating the copy in both directions; no
CopyDone should be sent in this case. See [Streaming Replication
Protocol](#protocol-replication) for more information on the subprotocol
transmitted over copy-both mode.

The CopyInResponse, CopyOutResponse and CopyBothResponse messages
include fields that inform the frontend of the number of columns per row
and the format codes being used for each column. (As of the present
implementation, all columns in a given `COPY` operation will use the
same format, but the message design does not assume this.)

Asynchronous Operations {#protocol-async}
-----------------------

There are several cases in which the backend will send messages that are
not specifically prompted by the frontend\'s command stream. Frontends
must be prepared to deal with these messages at any time, even when not
engaged in a query. At minimum, one should check for these cases before
beginning to read a query response.

It is possible for NoticeResponse messages to be generated due to
outside activity; for example, if the database administrator commands a
"fast" database shutdown, the backend will send a NoticeResponse
indicating this fact before closing the connection. Accordingly,
frontends should always be prepared to accept and display NoticeResponse
messages, even when the connection is nominally idle.

ParameterStatus messages will be generated whenever the active value
changes for any of the parameters the backend believes the frontend
should know about. Most commonly this occurs in response to a `SET` SQL
command executed by the frontend, and this case is effectively
synchronous MDASH but it is also possible for parameter status changes
to occur because the administrator changed a configuration file and then
sent the SIGHUP signal to the server. Also, if a `SET` command is rolled
back, an appropriate ParameterStatus message will be generated to report
the current effective value.

At present there is a hard-wired set of parameters for which
ParameterStatus will be generated: they are `server_version`,
`server_encoding`, `client_encoding`, `application_name`,
`default_transaction_read_only`, `in_hot_standby`, `is_superuser`,
`session_authorization`, `DateStyle`, `IntervalStyle`, `TimeZone`,
`integer_datetimes`, and `standard_conforming_strings`.
(`server_encoding`, `TimeZone`, and `integer_datetimes` were not
reported by releases before 8.0; `standard_conforming_strings` was not
reported by releases before 8.1; `IntervalStyle` was not reported by
releases before 8.4; `application_name` was not reported by releases
before 9.0; `default_transaction_read_only` and `in_hot_standby` were
not reported by releases before 14.) Note that `server_version`,
`server_encoding` and `integer_datetimes` are pseudo-parameters that
cannot change after startup. This set might change in the future, or
even become configurable. Accordingly, a frontend should simply ignore
ParameterStatus for parameters that it does not understand or care
about.

If a frontend issues a `LISTEN` command, then the backend will send a
NotificationResponse message (not to be confused with NoticeResponse!)
whenever a `NOTIFY` command is executed for the same channel name.

> **Note**
>
> At present, NotificationResponse can only be sent outside a
> transaction, and thus it will not occur in the middle of a
> command-response series, though it might occur just before
> ReadyForQuery. It is unwise to design frontend logic that assumes
> that, however. Good practice is to be able to accept
> NotificationResponse at any point in the protocol.

Canceling Requests in Progress
------------------------------

During the processing of a query, the frontend might request
cancellation of the query. The cancel request is not sent directly on
the open connection to the backend for reasons of implementation
efficiency: we don\'t want to have the backend constantly checking for
new input from the frontend during query processing. Cancel requests
should be relatively infrequent, so we make them slightly cumbersome in
order to avoid a penalty in the normal case.

To issue a cancel request, the frontend opens a new connection to the
server and sends a CancelRequest message, rather than the StartupMessage
message that would ordinarily be sent across a new connection. The
server will process this request and then close the connection. For
security reasons, no direct reply is made to the cancel request message.

A CancelRequest message will be ignored unless it contains the same key
data (PID and secret key) passed to the frontend during connection
start-up. If the request matches the PID and secret key for a currently
executing backend, the processing of the current query is aborted. (In
the existing implementation, this is done by sending a special signal to
the backend process that is processing the query.)

The cancellation signal might or might not have any effect MDASH for
example, if it arrives after the backend has finished processing the
query, then it will have no effect. If the cancellation is effective, it
results in the current command being terminated early with an error
message.

The upshot of all this is that for reasons of both security and
efficiency, the frontend has no direct way to tell whether a cancel
request has succeeded. It must continue to wait for the backend to
respond to the query. Issuing a cancel simply improves the odds that the
current query will finish soon, and improves the odds that it will fail
with an error message instead of succeeding.

Since the cancel request is sent across a new connection to the server
and not across the regular frontend/backend communication link, it is
possible for the cancel request to be issued by any process, not just
the frontend whose query is to be canceled. This might provide
additional flexibility when building multiple-process applications. It
also introduces a security risk, in that unauthorized persons might try
to cancel queries. The security risk is addressed by requiring a
dynamically generated secret key to be supplied in cancel requests.

Termination
-----------

The normal, graceful termination procedure is that the frontend sends a
Terminate message and immediately closes the connection. On receipt of
this message, the backend closes the connection and terminates.

In rare cases (such as an administrator-commanded database shutdown) the
backend might disconnect without any frontend request to do so. In such
cases the backend will attempt to send an error or notice message giving
the reason for the disconnection before it closes the connection.

Other termination scenarios arise from various failure cases, such as
core dump at one end or the other, loss of the communications link, loss
of message-boundary synchronization, etc. If either frontend or backend
sees an unexpected closure of the connection, it should clean up and
terminate. The frontend has the option of launching a new backend by
recontacting the server if it doesn\'t want to terminate itself. Closing
the connection is also advisable if an unrecognizable message type is
received, since this probably indicates loss of message-boundary sync.

For either normal or abnormal termination, any open transaction is
rolled back, not committed. One should note however that if a frontend
disconnects while a non-`SELECT` query is being processed, the backend
will probably finish the query before noticing the disconnection. If the
query is outside any transaction block (`BEGIN` \... `COMMIT` sequence)
then its results might be committed before the disconnection is
recognized.

SSL Session Encryption
----------------------

If PostgreSQL was built with SSL support, frontend/backend
communications can be encrypted using SSL. This provides communication
security in environments where attackers might be able to capture the
session traffic. For more information on encrypting PostgreSQL sessions
with SSL, see [???](#ssl-tcp).

To initiate an SSL-encrypted connection, the frontend initially sends an
SSLRequest message rather than a StartupMessage. The server then
responds with a single byte containing `S` or `N`, indicating that it is
willing or unwilling to perform SSL, respectively. The frontend might
close the connection at this point if it is dissatisfied with the
response. To continue after `S`, perform an SSL startup handshake (not
described here, part of the SSL specification) with the server. If this
is successful, continue with sending the usual StartupMessage. In this
case the StartupMessage and all subsequent data will be SSL-encrypted.
To continue after `N`, send the usual StartupMessage and proceed without
encryption. (Alternatively, it is permissible to issue a GSSENCRequest
message after an `N` response to try to use GSSAPI encryption instead of
SSL.)

The frontend should also be prepared to handle an ErrorMessage response
to SSLRequest from the server. This would only occur if the server
predates the addition of SSL support to PostgreSQL. (Such servers are
now very ancient, and likely do not exist in the wild anymore.) In this
case the connection must be closed, but the frontend might choose to
open a fresh connection and proceed without requesting SSL.

When SSL encryption can be performed, the server is expected to send
only the single `S` byte and then wait for the frontend to initiate an
SSL handshake. If additional bytes are available to read at this point,
it likely means that a man-in-the-middle is attempting to perform a
buffer-stuffing attack
([CVE-2021-23222](https://www.postgresql.org/support/security/CVE-2021-23222/)).
Frontends should be coded either to read exactly one byte from the
socket before turning the socket over to their SSL library, or to treat
it as a protocol violation if they find they have read additional bytes.

An initial SSLRequest can also be used in a connection that is being
opened to send a CancelRequest message.

While the protocol itself does not provide a way for the server to force
SSL encryption, the administrator can configure the server to reject
unencrypted sessions as a byproduct of authentication checking.

GSSAPI Session Encryption
-------------------------

If PostgreSQL was built with GSSAPI support, frontend/backend
communications can be encrypted using GSSAPI. This provides
communication security in environments where attackers might be able to
capture the session traffic. For more information on encrypting
PostgreSQL sessions with GSSAPI, see [???](#gssapi-enc).

To initiate a GSSAPI-encrypted connection, the frontend initially sends
a GSSENCRequest message rather than a StartupMessage. The server then
responds with a single byte containing `G` or `N`, indicating that it is
willing or unwilling to perform GSSAPI encryption, respectively. The
frontend might close the connection at this point if it is dissatisfied
with the response. To continue after `G`, using the GSSAPI C bindings as
discussed in [RFC 2744](https://tools.ietf.org/html/rfc2744) or
equivalent, perform a GSSAPI initialization by calling
`gss_init_sec_context()` in a loop and sending the result to the server,
starting with an empty input and then with each result from the server,
until it returns no output. When sending the results of
`gss_init_sec_context()` to the server, prepend the length of the
message as a four byte integer in network byte order. To continue after
`N`, send the usual StartupMessage and proceed without encryption.
(Alternatively, it is permissible to issue an SSLRequest message after
an `N` response to try to use SSL encryption instead of GSSAPI.)

The frontend should also be prepared to handle an ErrorMessage response
to GSSENCRequest from the server. This would only occur if the server
predates the addition of GSSAPI encryption support to PostgreSQL. In
this case the connection must be closed, but the frontend might choose
to open a fresh connection and proceed without requesting GSSAPI
encryption.

When GSSAPI encryption can be performed, the server is expected to send
only the single `G` byte and then wait for the frontend to initiate a
GSSAPI handshake. If additional bytes are available to read at this
point, it likely means that a man-in-the-middle is attempting to perform
a buffer-stuffing attack
([CVE-2021-23222](https://www.postgresql.org/support/security/CVE-2021-23222/)).
Frontends should be coded either to read exactly one byte from the
socket before turning the socket over to their GSSAPI library, or to
treat it as a protocol violation if they find they have read additional
bytes.

An initial GSSENCRequest can also be used in a connection that is being
opened to send a CancelRequest message.

Once GSSAPI encryption has been successfully established, use
`gss_wrap()` to encrypt the usual StartupMessage and all subsequent
data, prepending the length of the result from `gss_wrap()` as a four
byte integer in network byte order to the actual encrypted payload. Note
that the server will only accept encrypted packets from the client which
are less than 16kB; `gss_wrap_size_limit()` should be used by the client
to determine the size of the unencrypted message which will fit within
this limit and larger messages should be broken up into multiple
`gss_wrap()` calls. Typical segments are 8kB of unencrypted data,
resulting in encrypted packets of slightly larger than 8kB but well
within the 16kB maximum. The server can be expected to not send
encrypted packets of larger than 16kB to the client.

While the protocol itself does not provide a way for the server to force
GSSAPI encryption, the administrator can configure the server to reject
unencrypted sessions as a byproduct of authentication checking.

SASL Authentication
===================

SASL is a framework for authentication in connection-oriented protocols.
At the moment, PostgreSQL implements two SASL authentication mechanisms,
SCRAM-SHA-256 and SCRAM-SHA-256-PLUS. More might be added in the future.
The below steps illustrate how SASL authentication is performed in
general, while the next subsection gives more details on SCRAM-SHA-256
and SCRAM-SHA-256-PLUS.

To begin a SASL authentication exchange, the server sends an
AuthenticationSASL message. It includes a list of SASL authentication
mechanisms that the server can accept, in the server\'s preferred order.

The client selects one of the supported mechanisms from the list, and
sends a SASLInitialResponse message to the server. The message includes
the name of the selected mechanism, and an optional Initial Client
Response, if the selected mechanism uses that.

One or more server-challenge and client-response message will follow.
Each server-challenge is sent in an AuthenticationSASLContinue message,
followed by a response from client in a SASLResponse message. The
particulars of the messages are mechanism specific.

Finally, when the authentication exchange is completed successfully, the
server sends an AuthenticationSASLFinal message, followed immediately by
an AuthenticationOk message. The AuthenticationSASLFinal contains
additional server-to-client data, whose content is particular to the
selected authentication mechanism. If the authentication mechanism
doesn\'t use additional data that\'s sent at completion, the
AuthenticationSASLFinal message is not sent.

On error, the server can abort the authentication at any stage, and send
an ErrorMessage.

SCRAM-SHA-256 Authentication {#sasl-scram-sha-256}
----------------------------

The implemented SASL mechanisms at the moment are `SCRAM-SHA-256` and
its variant with channel binding `SCRAM-SHA-256-PLUS`. They are
described in detail in [RFC 7677](https://tools.ietf.org/html/rfc7677)
and [RFC 5802](https://tools.ietf.org/html/rfc5802).

When SCRAM-SHA-256 is used in PostgreSQL, the server will ignore the
user name that the client sends in the client-first-message. The user
name that was already sent in the startup message is used instead.
PostgreSQL supports multiple character encodings, while SCRAM dictates
UTF-8 to be used for the user name, so it might be impossible to
represent the PostgreSQL user name in UTF-8.

The SCRAM specification dictates that the password is also in UTF-8, and
is processed with the SASLprep algorithm. PostgreSQL, however, does not
require UTF-8 to be used for the password. When a user\'s password is
set, it is processed with SASLprep as if it was in UTF-8, regardless of
the actual encoding used. However, if it is not a legal UTF-8 byte
sequence, or it contains UTF-8 byte sequences that are prohibited by the
SASLprep algorithm, the raw password will be used without SASLprep
processing, instead of throwing an error. This allows the password to be
normalized when it is in UTF-8, but still allows a non-UTF-8 password to
be used, and doesn\'t require the system to know which encoding the
password is in.

Channel binding is supported in PostgreSQL builds with SSL support. The
SASL mechanism name for SCRAM with channel binding is
`SCRAM-SHA-256-PLUS`. The channel binding type used by PostgreSQL is
`tls-server-end-point`.

In SCRAM without channel binding, the server chooses a random number
that is transmitted to the client to be mixed with the user-supplied
password in the transmitted password hash. While this prevents the
password hash from being successfully retransmitted in a later session,
it does not prevent a fake server between the real server and client
from passing through the server\'s random value and successfully
authenticating.

SCRAM with channel binding prevents such man-in-the-middle attacks by
mixing the signature of the server\'s certificate into the transmitted
password hash. While a fake server can retransmit the real server\'s
certificate, it doesn\'t have access to the private key matching that
certificate, and therefore cannot prove it is the owner, causing SSL
connection failure.

The server sends an AuthenticationSASL message. It includes a list of
SASL authentication mechanisms that the server can accept. This will be
`SCRAM-SHA-256-PLUS` and `SCRAM-SHA-256` if the server is built with SSL
support, or else just the latter.

The client responds by sending a SASLInitialResponse message, which
indicates the chosen mechanism, `SCRAM-SHA-256` or `SCRAM-SHA-256-PLUS`.
(A client is free to choose either mechanism, but for better security it
should choose the channel-binding variant if it can support it.) In the
Initial Client response field, the message contains the SCRAM
client-first-message. The client-first-message also contains the channel
binding type chosen by the client.

Server sends an AuthenticationSASLContinue message, with a SCRAM
server-first-message as the content.

Client sends a SASLResponse message, with SCRAM client-final-message as
the content.

Server sends an AuthenticationSASLFinal message, with the SCRAM
server-final-message, followed immediately by an AuthenticationOk
message.

Streaming Replication Protocol {#protocol-replication}
==============================

To initiate streaming replication, the frontend sends the `replication`
parameter in the startup message. A Boolean value of `true` (or `on`,
`yes`, `1`) tells the backend to go into physical replication walsender
mode, wherein a small set of replication commands, shown below, can be
issued instead of SQL statements.

Passing `database` as the value for the `replication` parameter
instructs the backend to go into logical replication walsender mode,
connecting to the database specified in the `dbname` parameter. In
logical replication walsender mode, the replication commands shown below
as well as normal SQL commands can be issued.

In either physical replication or logical replication walsender mode,
only the simple query protocol can be used.

For the purpose of testing replication commands, you can make a
replication connection via psql or any other libpq-using tool with a
connection string including the `replication` option, e.g.:

    psql "dbname=postgres replication=database" -c "IDENTIFY_SYSTEM;"

However, it is often more useful to use [???](#app-pgreceivewal) (for
physical replication) or [???](#app-pgrecvlogical) (for logical
replication).

Replication commands are logged in the server log when
[???](#guc-log-replication-commands) is enabled.

The commands accepted in replication mode are:

`IDENTIFY_SYSTEM` IDENTIFY\_SYSTEM

:   Requests the server to identify itself. Server replies with a result
    set of a single row, containing four fields:

    `systemid` (`text`)

    :   The unique system identifier identifying the cluster. This can
        be used to check that the base backup used to initialize the
        standby came from the same cluster.

    `timeline` (`int4`)

    :   Current timeline ID. Also useful to check that the standby is
        consistent with the primary.

    `xlogpos` (`text`)

    :   Current WAL flush location. Useful to get a known location in
        the write-ahead log where streaming can start.

    `dbname` (`text`)

    :   Database connected to or null.

`SHOW` name SHOW

:   Requests the server to send the current setting of a run-time
    parameter. This is similar to the SQL command [???](#sql-show).

    name

    :   The name of a run-time parameter. Available parameters are
        documented in [???](#runtime-config).

`TIMELINE_HISTORY` tli TIMELINE\_HISTORY

:   Requests the server to send over the timeline history file for
    timeline tli. Server replies with a result set of a single row,
    containing two fields. While the fields are labeled as `text`, they
    effectively return raw bytes, with no encoding conversion:

    `filename` (`text`)

    :   File name of the timeline history file, e.g.,
        `00000002.history`.

    `content` (`text`)

    :   Contents of the timeline history file.

`CREATE_REPLICATION_SLOT` slot\_name \[ `TEMPORARY` \] { `PHYSICAL` \| `LOGICAL` } \[ ( option \[, \...\] ) \] CREATE\_REPLICATION\_SLOT

:   Create a physical or logical replication slot. See
    [???](#streaming-replication-slots) for more about replication
    slots.

    slot\_name

    :   The name of the slot to create. Must be a valid replication slot
        name (see [???](#streaming-replication-slots-manipulation)).

    output\_plugin

    :   The name of the output plugin used for logical decoding (see
        [???](#logicaldecoding-output-plugin)).

    `TEMPORARY`

    :   Specify that this replication slot is a temporary one. Temporary
        slots are not saved to disk and are automatically dropped on
        error or when the session has finished.

    The following options are supported:

    `TWO_PHASE [ boolean ]`

    :   If true, this logical replication slot supports decoding of
        two-phase transactions. With this option, two-phase commands
        like `PREPARE TRANSACTION`, `COMMIT PREPARED` and
        `ROLLBACK PREPARED` are decoded and transmitted. The transaction
        will be decoded and transmitted at `PREPARE TRANSACTION` time.
        The default is false.

    `RESERVE_WAL [ boolean ]`

    :   If true, this physical replication slot reserves WAL
        immediately. Otherwise, WAL is only reserved upon connection
        from a streaming replication client. The default is false.

    `SNAPSHOT { 'export' | 'use' | 'nothing' }`

    :   Decides what to do with the snapshot created during logical slot
        initialization. `'export'`, which is the default, will export
        the snapshot for use in other sessions. This option can\'t be
        used inside a transaction. `'use'` will use the snapshot for the
        current transaction executing the command. This option must be
        used in a transaction, and `CREATE_REPLICATION_SLOT` must be the
        first command run in that transaction. Finally, `'nothing'` will
        just use the snapshot for logical decoding as normal but won\'t
        do anything else with it.

    In response to this command, the server will send a one-row result
    set containing the following fields:

    `slot_name` (`text`)

    :   The name of the newly-created replication slot.

    `consistent_point` (`text`)

    :   The WAL location at which the slot became consistent. This is
        the earliest location from which streaming can start on this
        replication slot.

    `snapshot_name` (`text`)

    :   The identifier of the snapshot exported by the command. The
        snapshot is valid until a new command is executed on this
        connection or the replication connection is closed. Null if the
        created slot is physical.

    `output_plugin` (`text`)

    :   The name of the output plugin used by the newly-created
        replication slot. Null if the created slot is physical.

`CREATE_REPLICATION_SLOT` slot\_name \[ `TEMPORARY` \] { `PHYSICAL` \[ `RESERVE_WAL` \] \| `LOGICAL` output\_plugin \[ `EXPORT_SNAPSHOT` \| `NOEXPORT_SNAPSHOT` \| `USE_SNAPSHOT` \| `TWO_PHASE` \] }

:   For compatibility with older releases, this alternative syntax for
    the `CREATE_REPLICATION_SLOT` command is still supported.

`READ_REPLICATION_SLOT` slot\_name READ\_REPLICATION\_SLOT

:   Read some information associated with a replication slot. Returns a
    tuple with `NULL` values if the replication slot does not exist.
    This command is currently only supported for physical replication
    slots.

    In response to this command, the server will return a one-row result
    set, containing the following fields:

    `slot_type` (`text`)

    :   The replication slot\'s type, either `physical` or `NULL`.

    `restart_lsn` (`text`)

    :   The replication slot\'s `restart_lsn`.

    `restart_tli` (`int8`)

    :   The timeline ID associated with `restart_lsn`, following the
        current timeline history.

`START_REPLICATION` \[ `SLOT` slot\_name \] \[ `PHYSICAL` \] XXX/XXX \[ `TIMELINE` tli \] START\_REPLICATION

:   Instructs server to start streaming WAL, starting at WAL location
    XXX/XXX. If `TIMELINE` option is specified, streaming starts on
    timeline tli; otherwise, the server\'s current timeline is selected.
    The server can reply with an error, for example if the requested
    section of WAL has already been recycled. On success, the server
    responds with a CopyBothResponse message, and then starts to stream
    WAL to the frontend.

    If a slot\'s name is provided via slot\_name, it will be updated as
    replication progresses so that the server knows which WAL segments,
    and if `hot_standby_feedback` is on which transactions, are still
    needed by the standby.

    If the client requests a timeline that\'s not the latest but is part
    of the history of the server, the server will stream all the WAL on
    that timeline starting from the requested start point up to the
    point where the server switched to another timeline. If the client
    requests streaming at exactly the end of an old timeline, the server
    skips COPY mode entirely.

    After streaming all the WAL on a timeline that is not the latest
    one, the server will end streaming by exiting the COPY mode. When
    the client acknowledges this by also exiting COPY mode, the server
    sends a result set with one row and two columns, indicating the next
    timeline in this server\'s history. The first column is the next
    timeline\'s ID (type `int8`), and the second column is the WAL
    location where the switch happened (type `text`). Usually, the
    switch position is the end of the WAL that was streamed, but there
    are corner cases where the server can send some WAL from the old
    timeline that it has not itself replayed before promoting. Finally,
    the server sends two CommandComplete messages (one that ends the
    CopyData and the other ends the `START_REPLICATION` itself), and is
    ready to accept a new command.

    WAL data is sent as a series of CopyData messages. (This allows
    other information to be intermixed; in particular the server can
    send an ErrorResponse message if it encounters a failure after
    beginning to stream.) The payload of each CopyData message from
    server to the client contains a message of one of the following
    formats:

    XLogData (B)

    :   

        Byte1(\'w\')

        :   Identifies the message as WAL data.

        Int64

        :   The starting point of the WAL data in this message.

        Int64

        :   The current end of WAL on the server.

        Int64

        :   The server\'s system clock at the time of transmission, as
            microseconds since midnight on 2000-01-01.

        Byten

        :   A section of the WAL data stream.

            A single WAL record is never split across two XLogData
            messages. When a WAL record crosses a WAL page boundary, and
            is therefore already split using continuation records, it
            can be split at the page boundary. In other words, the first
            main WAL record and its continuation records can be sent in
            different XLogData messages.

    Primary keepalive message (B)

    :   

        Byte1(\'k\')

        :   Identifies the message as a sender keepalive.

        Int64

        :   The current end of WAL on the server.

        Int64

        :   The server\'s system clock at the time of transmission, as
            microseconds since midnight on 2000-01-01.

        Byte1

        :   1 means that the client should reply to this message as soon
            as possible, to avoid a timeout disconnect. 0 otherwise.

    The receiving process can send replies back to the sender at any
    time, using one of the following message formats (also in the
    payload of a CopyData message):

    Standby status update (F)

    :   

        Byte1(\'r\')

        :   Identifies the message as a receiver status update.

        Int64

        :   The location of the last WAL byte + 1 received and written
            to disk in the standby.

        Int64

        :   The location of the last WAL byte + 1 flushed to disk in the
            standby.

        Int64

        :   The location of the last WAL byte + 1 applied in the
            standby.

        Int64

        :   The client\'s system clock at the time of transmission, as
            microseconds since midnight on 2000-01-01.

        Byte1

        :   If 1, the client requests the server to reply to this
            message immediately. This can be used to ping the server, to
            test if the connection is still healthy.

    Hot standby feedback message (F)

    :   

        Byte1(\'h\')

        :   Identifies the message as a hot standby feedback message.

        Int64

        :   The client\'s system clock at the time of transmission, as
            microseconds since midnight on 2000-01-01.

        Int32

        :   The standby\'s current global xmin, excluding the
            catalog\_xmin from any replication slots. If both this value
            and the following catalog\_xmin are 0 this is treated as a
            notification that hot standby feedback will no longer be
            sent on this connection. Later non-zero messages may
            reinitiate the feedback mechanism.

        Int32

        :   The epoch of the global xmin xid on the standby.

        Int32

        :   The lowest catalog\_xmin of any replication slots on the
            standby. Set to 0 if no catalog\_xmin exists on the standby
            or if hot standby feedback is being disabled.

        Int32

        :   The epoch of the catalog\_xmin xid on the standby.

`START_REPLICATION` `SLOT` slot\_name `LOGICAL` XXX/XXX \[ ( option\_name \[ option\_value \] \[, \...\] ) \]

:   Instructs server to start streaming WAL for logical replication,
    starting at either WAL location XXX/XXX or the slot\'s
    `confirmed_flush_lsn` (see [???](#view-pg-replication-slots)),
    whichever is greater. This behavior makes it easier for clients to
    avoid updating their local LSN status when there is no data to
    process. However, starting at a different LSN than requested might
    not catch certain kinds of client errors; so the client may wish to
    check that `confirmed_flush_lsn` matches its expectations before
    issuing `START_REPLICATION`.

    The server can reply with an error, for example if the slot does not
    exist. On success, the server responds with a CopyBothResponse
    message, and then starts to stream WAL to the frontend.

    The messages inside the CopyBothResponse messages are of the same
    format documented for `START_REPLICATION ... PHYSICAL`, including
    two CommandComplete messages.

    The output plugin associated with the selected slot is used to
    process the output for streaming.

    `SLOT` slot\_name

    :   The name of the slot to stream changes from. This parameter is
        required, and must correspond to an existing logical replication
        slot created with `CREATE_REPLICATION_SLOT` in `LOGICAL` mode.

    XXX/XXX

    :   The WAL location to begin streaming at.

    option\_name

    :   The name of an option passed to the slot\'s logical decoding
        plugin.

    option\_value

    :   Optional value, in the form of a string constant, associated
        with the specified option.

`DROP_REPLICATION_SLOT` slot\_name \[`WAIT`\] DROP\_REPLICATION\_SLOT

:   Drops a replication slot, freeing any reserved server-side
    resources. If the slot is a logical slot that was created in a
    database other than the database the walsender is connected to, this
    command fails.

    slot\_name

    :   The name of the slot to drop.

    `WAIT`

    :   This option causes the command to wait if the slot is active
        until it becomes inactive, instead of the default behavior of
        raising an error.

`BASE_BACKUP` \[ ( option \[, \...\] ) \] BASE\_BACKUP

:   Instructs the server to start streaming a base backup. The system
    will automatically be put in backup mode before the backup is
    started, and taken out of it when the backup is complete. The
    following options are accepted:

    `LABEL` \'label\'

    :   Sets the label of the backup. If none is specified, a backup
        label of `base backup` will be used. The quoting rules for the
        label are the same as a standard SQL string with
        [???](#guc-standard-conforming-strings) turned on.

    `TARGET` \'target\'

    :   Tells the server where to send the backup. If the target is
        `client`, which is the default, the backup data is sent to the
        client. If it is `server`, the backup data is written to the
        server at the pathname specified by the `TARGET_DETAIL` option.
        If it is `blackhole`, the backup data is not sent anywhere; it
        is simply discarded.

        The `server` target requires superuser privilege or being
        granted the `pg_write_server_files` role.

    `TARGET_DETAIL` \'detail\'

    :   Provides additional information about the backup target.

        Currently, this option can only be used when the backup target
        is `server`. It specifies the server directory to which the
        backup should be written.

    `PROGRESS [ boolean ]`

    :   If set to true, request information required to generate a
        progress report. This will send back an approximate size in the
        header of each tablespace, which can be used to calculate how
        far along the stream is done. This is calculated by enumerating
        all the file sizes once before the transfer is even started, and
        might as such have a negative impact on the performance. In
        particular, it might take longer before the first data is
        streamed. Since the database files can change during the backup,
        the size is only approximate and might both grow and shrink
        between the time of approximation and the sending of the actual
        files. The default is false.

    `CHECKPOINT { 'fast' | 'spread' }`

    :   Sets the type of checkpoint to be performed at the beginning of
        the base backup. The default is `spread`.

    `WAL [ boolean ]`

    :   If set to true, include the necessary WAL segments in the
        backup. This will include all the files between start and stop
        backup in the `pg_wal` directory of the base directory tar file.
        The default is false.

    `WAIT [ boolean ]`

    :   If set to true, the backup will wait until the last required WAL
        segment has been archived, or emit a warning if log archiving is
        not enabled. If false, the backup will neither wait nor warn,
        leaving the client responsible for ensuring the required log is
        available. The default is true.

    `COMPRESSION` \'method\'

    :   Instructs the server to compress the backup using the specified
        method. Currently, the supported methods are `gzip`, `lz4`, and
        `zstd`.

    `COMPRESSION_DETAIL` detail

    :   Specifies details for the chosen compression method. This should
        only be used in conjunction with the `COMPRESSION` option. If
        the value is an integer, it specifies the compression level.
        Otherwise, it should be a comma-separated list of items, each of
        the form `keyword` or `keyword=value`. Currently, the supported
        keywords are `level` and `workers`.

        The `level` keyword sets the compression level. For `gzip` the
        compression level should be an integer between 1 and 9, for
        `lz4` an integer between 1 and 12, and for `zstd` an integer
        between 1 and 22.

        The `workers` keyword sets the number of threads that should be
        used for parallel compression. Parallel compression is supported
        only for `zstd`.

    `MAX_RATE` rate

    :   Limit (throttle) the maximum amount of data transferred from
        server to client per unit of time. The expected unit is
        kilobytes per second. If this option is specified, the value
        must either be equal to zero or it must fall within the range
        from 32 kB through 1 GB (inclusive). If zero is passed or the
        option is not specified, no restriction is imposed on the
        transfer.

    `TABLESPACE_MAP [ boolean ]`

    :   If true, include information about symbolic links present in the
        directory `pg_tblspc` in a file named `tablespace_map`. The
        tablespace map file includes each symbolic link name as it
        exists in the directory `pg_tblspc/` and the full path of that
        symbolic link. The default is false.

    `VERIFY_CHECKSUMS [ boolean ]`

    :   If true, checksums are verified during a base backup if they are
        enabled. If false, this is skipped. The default is true.

    `MANIFEST` manifest\_option

    :   When this option is specified with a value of `yes` or
        `force-encode`, a backup manifest is created and sent along with
        the backup. The manifest is a list of every file present in the
        backup with the exception of any WAL files that may be included.
        It also stores the size, last modification time, and optionally
        a checksum for each file. A value of `force-encode` forces all
        filenames to be hex-encoded; otherwise, this type of encoding is
        performed only for files whose names are non-UTF8 octet
        sequences. `force-encode` is intended primarily for testing
        purposes, to be sure that clients which read the backup manifest
        can handle this case. For compatibility with previous releases,
        the default is `MANIFEST 'no'`.

    `MANIFEST_CHECKSUMS` checksum\_algorithm

    :   Specifies the checksum algorithm that should be applied to each
        file included in the backup manifest. Currently, the available
        algorithms are `NONE`, `CRC32C`, `SHA224`, `SHA256`, `SHA384`,
        and `SHA512`. The default is `CRC32C`.

    When the backup is started, the server will first send two ordinary
    result sets, followed by one or more CopyOutResponse results.

    The first ordinary result set contains the starting position of the
    backup, in a single row with two columns. The first column contains
    the start position given in XLogRecPtr format, and the second column
    contains the corresponding timeline ID.

    The second ordinary result set has one row for each tablespace. The
    fields in this row are:

    `spcoid` (`oid`)

    :   The OID of the tablespace, or null if it\'s the base directory.

    `spclocation` (`text`)

    :   The full path of the tablespace directory, or null if it\'s the
        base directory.

    `size` (`int8`)

    :   The approximate size of the tablespace, in kilobytes (1024
        bytes), if progress report has been requested; otherwise it\'s
        null.

    After the second regular result set, a CopyOutResponse will be sent.
    The payload of each CopyData message will contain a message in one
    of the following formats:

    new archive (B)

    :   

        Byte1(\'n\')

        :   Identifes the messaage as indicating the start of a new
            archive. There will be one archive for the main data
            directory and one for each additional tablespace; each will
            use tar format (following the "ustar interchange format"
            specified in the POSIX 1003.1-2008 standard).

        String

        :   The file name for this archive.

        String

        :   For the main data directory, an empty string. For other
            tablespaces, the full path to the directory from which this
            archive was created.

    manifest (B)

    :   

        Byte1(\'m\')

        :   Identifes the message as indicating the start of the backup
            manifest.

    archive or manifest data (B)

    :   

        Byte1(\'d\')

        :   Identifes the message as containing archive or manifest
            data.

        Byten

        :   Data bytes.

    progress report (B)

    :   

        Byte1(\'p\')

        :   Identifes the message as a progress report.

        Int64

        :   The number of bytes from the current tablespace for which
            processing has been completed.

    After the CopyOutResponse, or all such responses, have been sent, a
    final ordinary result set will be sent, containing the WAL end
    position of the backup, in the same format as the start position.

    The tar archive for the data directory and each tablespace will
    contain all files in the directories, regardless of whether they are
    PostgreSQL files or other files added to the same directory. The
    only excluded files are:

    -   `postmaster.pid`

    -   `postmaster.opts`

    -   `pg_internal.init` (found in multiple directories)

    -   Various temporary files and directories created during the
        operation of the PostgreSQL server, such as any file or
        directory beginning with `pgsql_tmp` and temporary relations.

    -   Unlogged relations, except for the init fork which is required
        to recreate the (empty) unlogged relation on recovery.

    -   `pg_wal`, including subdirectories. If the backup is run with
        WAL files included, a synthesized version of `pg_wal` will be
        included, but it will only contain the files necessary for the
        backup to work, not the rest of the contents.

    -   `pg_dynshmem`, `pg_notify`, `pg_replslot`, `pg_serial`,
        `pg_snapshots`, `pg_stat_tmp`, and `pg_subtrans` are copied as
        empty directories (even if they are symbolic links).

    -   Files other than regular files and directories, such as symbolic
        links (other than for the directories listed above) and special
        device files, are skipped. (Symbolic links in `pg_tblspc` are
        maintained.)

    Owner, group, and file mode are set if the underlying file system on
    the server supports it.

Logical Streaming Replication Protocol {#protocol-logical-replication}
======================================

This section describes the logical replication protocol, which is the
message flow started by the `START_REPLICATION` `SLOT` slot\_name
`LOGICAL` replication command.

The logical streaming replication protocol builds on the primitives of
the physical streaming replication protocol.

Logical Streaming Replication Parameters {#protocol-logical-replication-params}
----------------------------------------

The logical replication `START_REPLICATION` command accepts following
parameters:

proto\_version

:   Protocol version. Currently versions `1`, `2`, and `3` are
    supported.

    Version `2` is supported only for server version 14 and above, and
    it allows streaming of large in-progress transactions.

    Version `3` is supported only for server version 15 and above, and
    it allows streaming of two-phase transactions.

publication\_names

:   Comma separated list of publication names for which to subscribe
    (receive changes). The individual publication names are treated as
    standard objects names and can be quoted the same as needed.

Logical Replication Protocol Messages {#protocol-logical-messages}
-------------------------------------

The individual protocol messages are discussed in the following
subsections. Individual messages are described in [Logical Replication
Message Formats](#protocol-logicalrep-message-formats).

All top-level protocol messages begin with a message type byte. While
represented in code as a character, this is a signed byte with no
associated encoding.

Since the streaming replication protocol supplies a message length there
is no need for top-level protocol messages to embed a length in their
header.

Logical Replication Protocol Message Flow {#protocol-logical-messages-flow}
-----------------------------------------

With the exception of the `START_REPLICATION` command and the replay
progress messages, all information flows only from the backend to the
frontend.

The logical replication protocol sends individual transactions one by
one. This means that all messages between a pair of Begin and Commit
messages belong to the same transaction. Similarly, all messages between
a pair of Begin Prepare and Prepare messages belong to the same
transaction. It also sends changes of large in-progress transactions
between a pair of Stream Start and Stream Stop messages. The last stream
of such a transaction contains a Stream Commit or Stream Abort message.

Every sent transaction contains zero or more DML messages (Insert,
Update, Delete). In case of a cascaded setup it can also contain Origin
messages. The origin message indicates that the transaction originated
on different replication node. Since a replication node in the scope of
logical replication protocol can be pretty much anything, the only
identifier is the origin name. It\'s downstream\'s responsibility to
handle this as needed (if needed). The Origin message is always sent
before any DML messages in the transaction.

Every DML message contains a relation OID, identifying the publisher\'s
relation that was acted on. Before the first DML message for a given
relation OID, a Relation message will be sent, describing the schema of
that relation. Subsequently, a new Relation message will be sent if the
relation\'s definition has changed since the last Relation message was
sent for it. (The protocol assumes that the client is capable of
remembering this metadata for as many relations as needed.)

Relation messages identify column types by their OIDs. In the case of a
built-in type, it is assumed that the client can look up that type OID
locally, so no additional data is needed. For a non-built-in type OID, a
Type message will be sent before the Relation message, to provide the
type name associated with that OID. Thus, a client that needs to
specifically identify the types of relation columns should cache the
contents of Type messages, and first consult that cache to see if the
type OID is defined there. If not, look up the type OID locally.

Message Data Types {#protocol-message-types}
==================

This section describes the base data types used in messages.

Intn(i)

:   An n-bit integer in network byte order (most significant byte
    first). If i is specified it is the exact value that will appear,
    otherwise the value is variable. Eg. Int16, Int32(42).

Intn\[k\]

:   An array of k n-bit integers, each in network byte order. The array
    length k is always determined by an earlier field in the message.
    Eg. Int16\[M\].

String(s)

:   A null-terminated string (C-style string). There is no specific
    length limitation on strings. If s is specified it is the exact
    value that will appear, otherwise the value is variable. Eg. String,
    String(\"user\").

    > **Note**
    >
    > *There is no predefined limit* on the length of a string that can
    > be returned by the backend. Good coding strategy for a frontend is
    > to use an expandable buffer so that anything that fits in memory
    > can be accepted. If that\'s not feasible, read the full string and
    > discard trailing characters that don\'t fit into your fixed-size
    > buffer.

Byten(c)

:   Exactly n bytes. If the field width n is not a constant, it is
    always determinable from an earlier field in the message. If c is
    specified it is the exact value. Eg. Byte2, Byte1(\'\\n\').

Message Formats {#protocol-message-formats}
===============

This section describes the detailed format of each message. Each is
marked to indicate that it can be sent by a frontend (F), a backend (B),
or both (F & B). Notice that although each message includes a byte count
at the beginning, the message format is defined so that the message end
can be found without reference to the byte count. This aids validity
checking. (The CopyData message is an exception, because it forms part
of a data stream; the contents of any individual CopyData message cannot
be interpretable on their own.)

AuthenticationOk (B)

:   

    Byte1(\'R\')

    :   Identifies the message as an authentication request.

    Int32(8)

    :   Length of message contents in bytes, including self.

    Int32(0)

    :   Specifies that the authentication was successful.

AuthenticationKerberosV5 (B)

:   

    Byte1(\'R\')

    :   Identifies the message as an authentication request.

    Int32(8)

    :   Length of message contents in bytes, including self.

    Int32(2)

    :   Specifies that Kerberos V5 authentication is required.

AuthenticationCleartextPassword (B)

:   

    Byte1(\'R\')

    :   Identifies the message as an authentication request.

    Int32(8)

    :   Length of message contents in bytes, including self.

    Int32(3)

    :   Specifies that a clear-text password is required.

AuthenticationMD5Password (B)

:   

    Byte1(\'R\')

    :   Identifies the message as an authentication request.

    Int32(12)

    :   Length of message contents in bytes, including self.

    Int32(5)

    :   Specifies that an MD5-encrypted password is required.

    Byte4

    :   The salt to use when encrypting the password.

AuthenticationSCMCredential (B)

:   

    Byte1(\'R\')

    :   Identifies the message as an authentication request.

    Int32(8)

    :   Length of message contents in bytes, including self.

    Int32(6)

    :   Specifies that an SCM credentials message is required.

AuthenticationGSS (B)

:   

    Byte1(\'R\')

    :   Identifies the message as an authentication request.

    Int32(8)

    :   Length of message contents in bytes, including self.

    Int32(7)

    :   Specifies that GSSAPI authentication is required.

AuthenticationGSSContinue (B)

:   

    Byte1(\'R\')

    :   Identifies the message as an authentication request.

    Int32

    :   Length of message contents in bytes, including self.

    Int32(8)

    :   Specifies that this message contains GSSAPI or SSPI data.

    Byten

    :   GSSAPI or SSPI authentication data.

AuthenticationSSPI (B)

:   

    Byte1(\'R\')

    :   Identifies the message as an authentication request.

    Int32(8)

    :   Length of message contents in bytes, including self.

    Int32(9)

    :   Specifies that SSPI authentication is required.

AuthenticationSASL (B)

:   

    Byte1(\'R\')

    :   Identifies the message as an authentication request.

    Int32

    :   Length of message contents in bytes, including self.

    Int32(10)

    :   Specifies that SASL authentication is required.

    The message body is a list of SASL authentication mechanisms, in the
    server\'s order of preference. A zero byte is required as terminator
    after the last authentication mechanism name. For each mechanism,
    there is the following:

    String

    :   Name of a SASL authentication mechanism.

AuthenticationSASLContinue (B)

:   

    Byte1(\'R\')

    :   Identifies the message as an authentication request.

    Int32

    :   Length of message contents in bytes, including self.

    Int32(11)

    :   Specifies that this message contains a SASL challenge.

    Byten

    :   SASL data, specific to the SASL mechanism being used.

AuthenticationSASLFinal (B)

:   

    Byte1(\'R\')

    :   Identifies the message as an authentication request.

    Int32

    :   Length of message contents in bytes, including self.

    Int32(12)

    :   Specifies that SASL authentication has completed.

    Byten

    :   SASL outcome \"additional data\", specific to the SASL mechanism
        being used.

BackendKeyData (B)

:   

    Byte1(\'K\')

    :   Identifies the message as cancellation key data. The frontend
        must save these values if it wishes to be able to issue
        CancelRequest messages later.

    Int32(12)

    :   Length of message contents in bytes, including self.

    Int32

    :   The process ID of this backend.

    Int32

    :   The secret key of this backend.

Bind (F)

:   

    Byte1(\'B\')

    :   Identifies the message as a Bind command.

    Int32

    :   Length of message contents in bytes, including self.

    String

    :   The name of the destination portal (an empty string selects the
        unnamed portal).

    String

    :   The name of the source prepared statement (an empty string
        selects the unnamed prepared statement).

    Int16

    :   The number of parameter format codes that follow (denoted C
        below). This can be zero to indicate that there are no
        parameters or that the parameters all use the default format
        (text); or one, in which case the specified format code is
        applied to all parameters; or it can equal the actual number of
        parameters.

    Int16\[C\]

    :   The parameter format codes. Each must presently be zero (text)
        or one (binary).

    Int16

    :   The number of parameter values that follow (possibly zero). This
        must match the number of parameters needed by the query.

    Next, the following pair of fields appear for each parameter:

    Int32

    :   The length of the parameter value, in bytes (this count does not
        include itself). Can be zero. As a special case, -1 indicates a
        NULL parameter value. No value bytes follow in the NULL case.

    Byten

    :   The value of the parameter, in the format indicated by the
        associated format code. n is the above length.

    After the last parameter, the following fields appear:

    Int16

    :   The number of result-column format codes that follow (denoted R
        below). This can be zero to indicate that there are no result
        columns or that the result columns should all use the default
        format (text); or one, in which case the specified format code
        is applied to all result columns (if any); or it can equal the
        actual number of result columns of the query.

    Int16\[R\]

    :   The result-column format codes. Each must presently be zero
        (text) or one (binary).

BindComplete (B)

:   

    Byte1(\'2\')

    :   Identifies the message as a Bind-complete indicator.

    Int32(4)

    :   Length of message contents in bytes, including self.

CancelRequest (F)

:   

    Int32(16)

    :   Length of message contents in bytes, including self.

    Int32(80877102)

    :   The cancel request code. The value is chosen to contain `1234`
        in the most significant 16 bits, and `5678` in the least
        significant 16 bits. (To avoid confusion, this code must not be
        the same as any protocol version number.)

    Int32

    :   The process ID of the target backend.

    Int32

    :   The secret key for the target backend.

Close (F)

:   

    Byte1(\'C\')

    :   Identifies the message as a Close command.

    Int32

    :   Length of message contents in bytes, including self.

    Byte1

    :   \'`S`\' to close a prepared statement; or \'`P`\' to close a
        portal.

    String

    :   The name of the prepared statement or portal to close (an empty
        string selects the unnamed prepared statement or portal).

CloseComplete (B)

:   

    Byte1(\'3\')

    :   Identifies the message as a Close-complete indicator.

    Int32(4)

    :   Length of message contents in bytes, including self.

CommandComplete (B)

:   

    Byte1(\'C\')

    :   Identifies the message as a command-completed response.

    Int32

    :   Length of message contents in bytes, including self.

    String

    :   The command tag. This is usually a single word that identifies
        which SQL command was completed.

        For an `INSERT` command, the tag is `INSERT oid
                rows`, where rows is the number of rows inserted. oid
        used to be the object ID of the inserted row if rows was 1 and
        the target table had OIDs, but OIDs system columns are not
        supported anymore; therefore oid is always 0.

        For a `DELETE` command, the tag is `DELETE rows` where rows is
        the number of rows deleted.

        For an `UPDATE` command, the tag is `UPDATE rows` where rows is
        the number of rows updated.

        For a `SELECT` or `CREATE TABLE AS` command, the tag is
        `SELECT rows` where rows is the number of rows retrieved.

        For a `MOVE` command, the tag is `MOVE rows` where rows is the
        number of rows the cursor\'s position has been changed by.

        For a `FETCH` command, the tag is `FETCH rows` where rows is the
        number of rows that have been retrieved from the cursor.

        For a `COPY` command, the tag is `COPY rows` where rows is the
        number of rows copied. (Note: the row count appears only in
        PostgreSQL 8.2 and later.)

CopyData (F & B)

:   

    Byte1(\'d\')

    :   Identifies the message as `COPY` data.

    Int32

    :   Length of message contents in bytes, including self.

    Byten

    :   Data that forms part of a `COPY` data stream. Messages sent from
        the backend will always correspond to single data rows, but
        messages sent by frontends might divide the data stream
        arbitrarily.

CopyDone (F & B)

:   

    Byte1(\'c\')

    :   Identifies the message as a `COPY`-complete indicator.

    Int32(4)

    :   Length of message contents in bytes, including self.

CopyFail (F)

:   

    Byte1(\'f\')

    :   Identifies the message as a `COPY`-failure indicator.

    Int32

    :   Length of message contents in bytes, including self.

    String

    :   An error message to report as the cause of failure.

CopyInResponse (B)

:   

    Byte1(\'G\')

    :   Identifies the message as a Start Copy In response. The frontend
        must now send copy-in data (if not prepared to do so, send a
        CopyFail message).

    Int32

    :   Length of message contents in bytes, including self.

    Int8

    :   0 indicates the overall `COPY` format is textual (rows separated
        by newlines, columns separated by separator characters, etc). 1
        indicates the overall copy format is binary (similar to DataRow
        format). See [???](#sql-copy) for more information.

    Int16

    :   The number of columns in the data to be copied (denoted N
        below).

    Int16\[N\]

    :   The format codes to be used for each column. Each must presently
        be zero (text) or one (binary). All must be zero if the overall
        copy format is textual.

CopyOutResponse (B)

:   

    Byte1(\'H\')

    :   Identifies the message as a Start Copy Out response. This
        message will be followed by copy-out data.

    Int32

    :   Length of message contents in bytes, including self.

    Int8

    :   0 indicates the overall `COPY` format is textual (rows separated
        by newlines, columns separated by separator characters, etc). 1
        indicates the overall copy format is binary (similar to DataRow
        format). See [???](#sql-copy) for more information.

    Int16

    :   The number of columns in the data to be copied (denoted N
        below).

    Int16\[N\]

    :   The format codes to be used for each column. Each must presently
        be zero (text) or one (binary). All must be zero if the overall
        copy format is textual.

CopyBothResponse (B)

:   

    Byte1(\'W\')

    :   Identifies the message as a Start Copy Both response. This
        message is used only for Streaming Replication.

    Int32

    :   Length of message contents in bytes, including self.

    Int8

    :   0 indicates the overall `COPY` format is textual (rows separated
        by newlines, columns separated by separator characters, etc). 1
        indicates the overall copy format is binary (similar to DataRow
        format). See [???](#sql-copy) for more information.

    Int16

    :   The number of columns in the data to be copied (denoted N
        below).

    Int16\[N\]

    :   The format codes to be used for each column. Each must presently
        be zero (text) or one (binary). All must be zero if the overall
        copy format is textual.

DataRow (B)

:   

    Byte1(\'D\')

    :   Identifies the message as a data row.

    Int32

    :   Length of message contents in bytes, including self.

    Int16

    :   The number of column values that follow (possibly zero).

    Next, the following pair of fields appear for each column:

    Int32

    :   The length of the column value, in bytes (this count does not
        include itself). Can be zero. As a special case, -1 indicates a
        NULL column value. No value bytes follow in the NULL case.

    Byten

    :   The value of the column, in the format indicated by the
        associated format code. n is the above length.

Describe (F)

:   

    Byte1(\'D\')

    :   Identifies the message as a Describe command.

    Int32

    :   Length of message contents in bytes, including self.

    Byte1

    :   \'`S`\' to describe a prepared statement; or \'`P`\' to describe
        a portal.

    String

    :   The name of the prepared statement or portal to describe (an
        empty string selects the unnamed prepared statement or portal).

EmptyQueryResponse (B)

:   

    Byte1(\'I\')

    :   Identifies the message as a response to an empty query string.
        (This substitutes for CommandComplete.)

    Int32(4)

    :   Length of message contents in bytes, including self.

ErrorResponse (B)

:   

    Byte1(\'E\')

    :   Identifies the message as an error.

    Int32

    :   Length of message contents in bytes, including self.

    The message body consists of one or more identified fields, followed
    by a zero byte as a terminator. Fields can appear in any order. For
    each field there is the following:

    Byte1

    :   A code identifying the field type; if zero, this is the message
        terminator and no string follows. The presently defined field
        types are listed in [Error and Notice Message
        Fields](#protocol-error-fields). Since more field types might be
        added in future, frontends should silently ignore fields of
        unrecognized type.

    String

    :   The field value.

Execute (F)

:   

    Byte1(\'E\')

    :   Identifies the message as an Execute command.

    Int32

    :   Length of message contents in bytes, including self.

    String

    :   The name of the portal to execute (an empty string selects the
        unnamed portal).

    Int32

    :   Maximum number of rows to return, if portal contains a query
        that returns rows (ignored otherwise). Zero denotes "no limit".

Flush (F)

:   

    Byte1(\'H\')

    :   Identifies the message as a Flush command.

    Int32(4)

    :   Length of message contents in bytes, including self.

FunctionCall (F)

:   

    Byte1(\'F\')

    :   Identifies the message as a function call.

    Int32

    :   Length of message contents in bytes, including self.

    Int32

    :   Specifies the object ID of the function to call.

    Int16

    :   The number of argument format codes that follow (denoted C
        below). This can be zero to indicate that there are no arguments
        or that the arguments all use the default format (text); or one,
        in which case the specified format code is applied to all
        arguments; or it can equal the actual number of arguments.

    Int16\[C\]

    :   The argument format codes. Each must presently be zero (text) or
        one (binary).

    Int16

    :   Specifies the number of arguments being supplied to the
        function.

    Next, the following pair of fields appear for each argument:

    Int32

    :   The length of the argument value, in bytes (this count does not
        include itself). Can be zero. As a special case, -1 indicates a
        NULL argument value. No value bytes follow in the NULL case.

    Byten

    :   The value of the argument, in the format indicated by the
        associated format code. n is the above length.

    After the last argument, the following field appears:

    Int16

    :   The format code for the function result. Must presently be zero
        (text) or one (binary).

FunctionCallResponse (B)

:   

    Byte1(\'V\')

    :   Identifies the message as a function call result.

    Int32

    :   Length of message contents in bytes, including self.

    Int32

    :   The length of the function result value, in bytes (this count
        does not include itself). Can be zero. As a special case, -1
        indicates a NULL function result. No value bytes follow in the
        NULL case.

    Byten

    :   The value of the function result, in the format indicated by the
        associated format code. n is the above length.

GSSENCRequest (F)

:   

    Int32(8)

    :   Length of message contents in bytes, including self.

    Int32(80877104)

    :   The GSSAPI Encryption request code. The value is chosen to
        contain `1234` in the most significant 16 bits, and `5680` in
        the least significant 16 bits. (To avoid confusion, this code
        must not be the same as any protocol version number.)

GSSResponse (F)

:   

    Byte1(\'p\')

    :   Identifies the message as a GSSAPI or SSPI response. Note that
        this is also used for SASL and password response messages. The
        exact message type can be deduced from the context.

    Int32

    :   Length of message contents in bytes, including self.

    Byten

    :   GSSAPI/SSPI specific message data.

NegotiateProtocolVersion (B)

:   

    Byte1(\'v\')

    :   Identifies the message as a protocol version negotiation
        message.

    Int32

    :   Length of message contents in bytes, including self.

    Int32

    :   Newest minor protocol version supported by the server for the
        major protocol version requested by the client.

    Int32

    :   Number of protocol options not recognized by the server.

    Then, for protocol option not recognized by the server, there is the
    following:

    String

    :   The option name.

NoData (B)

:   

    Byte1(\'n\')

    :   Identifies the message as a no-data indicator.

    Int32(4)

    :   Length of message contents in bytes, including self.

NoticeResponse (B)

:   

    Byte1(\'N\')

    :   Identifies the message as a notice.

    Int32

    :   Length of message contents in bytes, including self.

    The message body consists of one or more identified fields, followed
    by a zero byte as a terminator. Fields can appear in any order. For
    each field there is the following:

    Byte1

    :   A code identifying the field type; if zero, this is the message
        terminator and no string follows. The presently defined field
        types are listed in [Error and Notice Message
        Fields](#protocol-error-fields). Since more field types might be
        added in future, frontends should silently ignore fields of
        unrecognized type.

    String

    :   The field value.

NotificationResponse (B)

:   

    Byte1(\'A\')

    :   Identifies the message as a notification response.

    Int32

    :   Length of message contents in bytes, including self.

    Int32

    :   The process ID of the notifying backend process.

    String

    :   The name of the channel that the notify has been raised on.

    String

    :   The "payload" string passed from the notifying process.

ParameterDescription (B)

:   

    Byte1(\'t\')

    :   Identifies the message as a parameter description.

    Int32

    :   Length of message contents in bytes, including self.

    Int16

    :   The number of parameters used by the statement (can be zero).

    Then, for each parameter, there is the following:

    Int32

    :   Specifies the object ID of the parameter data type.

ParameterStatus (B)

:   

    Byte1(\'S\')

    :   Identifies the message as a run-time parameter status report.

    Int32

    :   Length of message contents in bytes, including self.

    String

    :   The name of the run-time parameter being reported.

    String

    :   The current value of the parameter.

Parse (F)

:   

    Byte1(\'P\')

    :   Identifies the message as a Parse command.

    Int32

    :   Length of message contents in bytes, including self.

    String

    :   The name of the destination prepared statement (an empty string
        selects the unnamed prepared statement).

    String

    :   The query string to be parsed.

    Int16

    :   The number of parameter data types specified (can be zero). Note
        that this is not an indication of the number of parameters that
        might appear in the query string, only the number that the
        frontend wants to prespecify types for.

    Then, for each parameter, there is the following:

    Int32

    :   Specifies the object ID of the parameter data type. Placing a
        zero here is equivalent to leaving the type unspecified.

ParseComplete (B)

:   

    Byte1(\'1\')

    :   Identifies the message as a Parse-complete indicator.

    Int32(4)

    :   Length of message contents in bytes, including self.

PasswordMessage (F)

:   

    Byte1(\'p\')

    :   Identifies the message as a password response. Note that this is
        also used for GSSAPI, SSPI and SASL response messages. The exact
        message type can be deduced from the context.

    Int32

    :   Length of message contents in bytes, including self.

    String

    :   The password (encrypted, if requested).

PortalSuspended (B)

:   

    Byte1(\'s\')

    :   Identifies the message as a portal-suspended indicator. Note
        this only appears if an Execute message\'s row-count limit was
        reached.

    Int32(4)

    :   Length of message contents in bytes, including self.

Query (F)

:   

    Byte1(\'Q\')

    :   Identifies the message as a simple query.

    Int32

    :   Length of message contents in bytes, including self.

    String

    :   The query string itself.

ReadyForQuery (B)

:   

    Byte1(\'Z\')

    :   Identifies the message type. ReadyForQuery is sent whenever the
        backend is ready for a new query cycle.

    Int32(5)

    :   Length of message contents in bytes, including self.

    Byte1

    :   Current backend transaction status indicator. Possible values
        are \'`I`\' if idle (not in a transaction block); \'`T`\' if in
        a transaction block; or \'`E`\' if in a failed transaction block
        (queries will be rejected until block is ended).

RowDescription (B)

:   

    Byte1(\'T\')

    :   Identifies the message as a row description.

    Int32

    :   Length of message contents in bytes, including self.

    Int16

    :   Specifies the number of fields in a row (can be zero).

    Then, for each field, there is the following:

    String

    :   The field name.

    Int32

    :   If the field can be identified as a column of a specific table,
        the object ID of the table; otherwise zero.

    Int16

    :   If the field can be identified as a column of a specific table,
        the attribute number of the column; otherwise zero.

    Int32

    :   The object ID of the field\'s data type.

    Int16

    :   The data type size (see `pg_type.typlen`). Note that negative
        values denote variable-width types.

    Int32

    :   The type modifier (see `pg_attribute.atttypmod`). The meaning of
        the modifier is type-specific.

    Int16

    :   The format code being used for the field. Currently will be zero
        (text) or one (binary). In a RowDescription returned from the
        statement variant of Describe, the format code is not yet known
        and will always be zero.

SASLInitialResponse (F)

:   

    Byte1(\'p\')

    :   Identifies the message as an initial SASL response. Note that
        this is also used for GSSAPI, SSPI and password response
        messages. The exact message type is deduced from the context.

    Int32

    :   Length of message contents in bytes, including self.

    String

    :   Name of the SASL authentication mechanism that the client
        selected.

    Int32

    :   Length of SASL mechanism specific \"Initial Client Response\"
        that follows, or -1 if there is no Initial Response.

    Byten

    :   SASL mechanism specific \"Initial Response\".

SASLResponse (F)

:   

    Byte1(\'p\')

    :   Identifies the message as a SASL response. Note that this is
        also used for GSSAPI, SSPI and password response messages. The
        exact message type can be deduced from the context.

    Int32

    :   Length of message contents in bytes, including self.

    Byten

    :   SASL mechanism specific message data.

SSLRequest (F)

:   

    Int32(8)

    :   Length of message contents in bytes, including self.

    Int32(80877103)

    :   The SSL request code. The value is chosen to contain `1234` in
        the most significant 16 bits, and `5679` in the least
        significant 16 bits. (To avoid confusion, this code must not be
        the same as any protocol version number.)

StartupMessage (F)

:   

    Int32

    :   Length of message contents in bytes, including self.

    Int32(196608)

    :   The protocol version number. The most significant 16 bits are
        the major version number (3 for the protocol described here).
        The least significant 16 bits are the minor version number (0
        for the protocol described here).

    The protocol version number is followed by one or more pairs of
    parameter name and value strings. A zero byte is required as a
    terminator after the last name/value pair. Parameters can appear in
    any order. `user` is required, others are optional. Each parameter
    is specified as:

    String

    :   The parameter name. Currently recognized names are:

        `user`

        :   The database user name to connect as. Required; there is no
            default.

        `database`

        :   The database to connect to. Defaults to the user name.

        `options`

        :   Command-line arguments for the backend. (This is deprecated
            in favor of setting individual run-time parameters.) Spaces
            within this string are considered to separate arguments,
            unless escaped with a backslash (`\`); write `\\` to
            represent a literal backslash.

        `replication`

        :   Used to connect in streaming replication mode, where a small
            set of replication commands can be issued instead of SQL
            statements. Value can be `true`, `false`, or `database`, and
            the default is `false`. See [Streaming Replication
            Protocol](#protocol-replication) for details.

        In addition to the above, other parameters may be listed.
        Parameter names beginning with `_pq_.` are reserved for use as
        protocol extensions, while others are treated as run-time
        parameters to be set at backend start time. Such settings will
        be applied during backend start (after parsing the command-line
        arguments if any) and will act as session defaults.

    String

    :   The parameter value.

Sync (F)

:   

    Byte1(\'S\')

    :   Identifies the message as a Sync command.

    Int32(4)

    :   Length of message contents in bytes, including self.

Terminate (F)

:   

    Byte1(\'X\')

    :   Identifies the message as a termination.

    Int32(4)

    :   Length of message contents in bytes, including self.

Error and Notice Message Fields {#protocol-error-fields}
===============================

This section describes the fields that can appear in ErrorResponse and
NoticeResponse messages. Each field type has a single-byte
identification token. Note that any given field type should appear at
most once per message.

`S`

:   Severity: the field contents are `ERROR`, `FATAL`, or `PANIC` (in an
    error message), or `WARNING`, `NOTICE`, `DEBUG`, `INFO`, or `LOG`
    (in a notice message), or a localized translation of one of these.
    Always present.

`V`

:   Severity: the field contents are `ERROR`, `FATAL`, or `PANIC` (in an
    error message), or `WARNING`, `NOTICE`, `DEBUG`, `INFO`, or `LOG`
    (in a notice message). This is identical to the `S` field except
    that the contents are never localized. This is present only in
    messages generated by PostgreSQL versions 9.6 and later.

`C`

:   Code: the SQLSTATE code for the error (see
    [???](#errcodes-appendix)). Not localizable. Always present.

`M`

:   Message: the primary human-readable error message. This should be
    accurate but terse (typically one line). Always present.

`D`

:   Detail: an optional secondary error message carrying more detail
    about the problem. Might run to multiple lines.

`H`

:   Hint: an optional suggestion what to do about the problem. This is
    intended to differ from Detail in that it offers advice (potentially
    inappropriate) rather than hard facts. Might run to multiple lines.

`P`

:   Position: the field value is a decimal ASCII integer, indicating an
    error cursor position as an index into the original query string.
    The first character has index 1, and positions are measured in
    characters not bytes.

`p`

:   Internal position: this is defined the same as the `P` field, but it
    is used when the cursor position refers to an internally generated
    command rather than the one submitted by the client. The `q` field
    will always appear when this field appears.

`q`

:   Internal query: the text of a failed internally-generated command.
    This could be, for example, an SQL query issued by a PL/pgSQL
    function.

`W`

:   Where: an indication of the context in which the error occurred.
    Presently this includes a call stack traceback of active procedural
    language functions and internally-generated queries. The trace is
    one entry per line, most recent first.

`s`

:   Schema name: if the error was associated with a specific database
    object, the name of the schema containing that object, if any.

`t`

:   Table name: if the error was associated with a specific table, the
    name of the table. (Refer to the schema name field for the name of
    the table\'s schema.)

`c`

:   Column name: if the error was associated with a specific table
    column, the name of the column. (Refer to the schema and table name
    fields to identify the table.)

`d`

:   Data type name: if the error was associated with a specific data
    type, the name of the data type. (Refer to the schema name field for
    the name of the data type\'s schema.)

`n`

:   Constraint name: if the error was associated with a specific
    constraint, the name of the constraint. Refer to fields listed above
    for the associated table or domain. (For this purpose, indexes are
    treated as constraints, even if they weren\'t created with
    constraint syntax.)

`F`

:   File: the file name of the source-code location where the error was
    reported.

`L`

:   Line: the line number of the source-code location where the error
    was reported.

`R`

:   Routine: the name of the source-code routine reporting the error.

> **Note**
>
> The fields for schema name, table name, column name, data type name,
> and constraint name are supplied only for a limited number of error
> types; see [???](#errcodes-appendix). Frontends should not assume that
> the presence of any of these fields guarantees the presence of another
> field. Core error sources observe the interrelationships noted above,
> but user-defined functions may use these fields in other ways. In the
> same vein, clients should not assume that these fields denote
> contemporary objects in the current database.

The client is responsible for formatting displayed information to meet
its needs; in particular it should break long lines as needed. Newline
characters appearing in the error message fields should be treated as
paragraph breaks, not line breaks.

Logical Replication Message Formats {#protocol-logicalrep-message-formats}
===================================

This section describes the detailed format of each logical replication
message. These messages are either returned by the replication slot SQL
interface or are sent by a walsender. In the case of a walsender, they
are encapsulated inside replication protocol WAL messages as described
in [Streaming Replication Protocol](#protocol-replication), and
generally obey the same message flow as physical replication.

Begin

:   

    Byte1(\'B\')

    :   Identifies the message as a begin message.

    Int64 (XLogRecPtr)

    :   The final LSN of the transaction.

    Int64 (TimestampTz)

    :   Commit timestamp of the transaction. The value is in number of
        microseconds since PostgreSQL epoch (2000-01-01).

    Int32 (TransactionId)

    :   Xid of the transaction.

Message

:   

    Byte1(\'M\')

    :   Identifies the message as a logical decoding message.

    Int32 (TransactionId)

    :   Xid of the transaction (only present for streamed transactions).
        This field is available since protocol version 2.

    Int8

    :   Flags; Either 0 for no flags or 1 if the logical decoding
        message is transactional.

    Int64 (XLogRecPtr)

    :   The LSN of the logical decoding message.

    String

    :   The prefix of the logical decoding message.

    Int32

    :   Length of the content.

    Byten

    :   The content of the logical decoding message.

Commit

:   

    Byte1(\'C\')

    :   Identifies the message as a commit message.

    Int8(0)

    :   Flags; currently unused.

    Int64 (XLogRecPtr)

    :   The LSN of the commit.

    Int64 (XLogRecPtr)

    :   The end LSN of the transaction.

    Int64 (TimestampTz)

    :   Commit timestamp of the transaction. The value is in number of
        microseconds since PostgreSQL epoch (2000-01-01).

Origin

:   

    Byte1(\'O\')

    :   Identifies the message as an origin message.

    Int64 (XLogRecPtr)

    :   The LSN of the commit on the origin server.

    String

    :   Name of the origin.

    Note that there can be multiple Origin messages inside a single
    transaction.

Relation

:   

    Byte1(\'R\')

    :   Identifies the message as a relation message.

    Int32 (TransactionId)

    :   Xid of the transaction (only present for streamed transactions).
        This field is available since protocol version 2.

    Int32 (Oid)

    :   OID of the relation.

    String

    :   Namespace (empty string for `pg_catalog`).

    String

    :   Relation name.

    Int8

    :   Replica identity setting for the relation (same as relreplident
        in pg\_class).

    Int16

    :   Number of columns.

    Next, the following message part appears for each column included in
    the publication (except generated columns):

    Int8

    :   Flags for the column. Currently can be either 0 for no flags or
        1 which marks the column as part of the key.

    String

    :   Name of the column.

    Int32 (Oid)

    :   OID of the column\'s data type.

    Int32

    :   Type modifier of the column (atttypmod).

Type

:   

    Byte1(\'Y\')

    :   Identifies the message as a type message.

    Int32 (TransactionId)

    :   Xid of the transaction (only present for streamed transactions).
        This field is available since protocol version 2.

    Int32 (Oid)

    :   OID of the data type.

    String

    :   Namespace (empty string for `pg_catalog`).

    String

    :   Name of the data type.

Insert

:   

    Byte1(\'I\')

    :   Identifies the message as an insert message.

    Int32 (TransactionId)

    :   Xid of the transaction (only present for streamed transactions).
        This field is available since protocol version 2.

    Int32 (Oid)

    :   OID of the relation corresponding to the ID in the relation
        message.

    Byte1(\'N\')

    :   Identifies the following TupleData message as a new tuple.

    TupleData

    :   TupleData message part representing the contents of new tuple.

Update

:   

    Byte1(\'U\')

    :   Identifies the message as an update message.

    Int32 (TransactionId)

    :   Xid of the transaction (only present for streamed transactions).
        This field is available since protocol version 2.

    Int32 (Oid)

    :   OID of the relation corresponding to the ID in the relation
        message.

    Byte1(\'K\')

    :   Identifies the following TupleData submessage as a key. This
        field is optional and is only present if the update changed data
        in any of the column(s) that are part of the REPLICA IDENTITY
        index.

    Byte1(\'O\')

    :   Identifies the following TupleData submessage as an old tuple.
        This field is optional and is only present if table in which the
        update happened has REPLICA IDENTITY set to FULL.

    TupleData

    :   TupleData message part representing the contents of the old
        tuple or primary key. Only present if the previous \'O\' or
        \'K\' part is present.

    Byte1(\'N\')

    :   Identifies the following TupleData message as a new tuple.

    TupleData

    :   TupleData message part representing the contents of a new tuple.

    The Update message may contain either a \'K\' message part or an
    \'O\' message part or neither of them, but never both of them.

Delete

:   

    Byte1(\'D\')

    :   Identifies the message as a delete message.

    Int32 (TransactionId)

    :   Xid of the transaction (only present for streamed transactions).
        This field is available since protocol version 2.

    Int32 (Oid)

    :   OID of the relation corresponding to the ID in the relation
        message.

    Byte1(\'K\')

    :   Identifies the following TupleData submessage as a key. This
        field is present if the table in which the delete has happened
        uses an index as REPLICA IDENTITY.

    Byte1(\'O\')

    :   Identifies the following TupleData message as an old tuple. This
        field is present if the table in which the delete happened has
        REPLICA IDENTITY set to FULL.

    TupleData

    :   TupleData message part representing the contents of the old
        tuple or primary key, depending on the previous field.

    The Delete message may contain either a \'K\' message part or an
    \'O\' message part, but never both of them.

Truncate

:   

    Byte1(\'T\')

    :   Identifies the message as a truncate message.

    Int32 (TransactionId)

    :   Xid of the transaction (only present for streamed transactions).
        This field is available since protocol version 2.

    Int32

    :   Number of relations

    Int8

    :   Option bits for `TRUNCATE`: 1 for `CASCADE`, 2 for
        `RESTART IDENTITY`

    Int32 (Oid)

    :   OID of the relation corresponding to the ID in the relation
        message. This field is repeated for each relation.

The following messages (Stream Start, Stream Stop, Stream Commit, and
Stream Abort) are available since protocol version 2.

Stream Start

:   

    Byte1(\'S\')

    :   Identifies the message as a stream start message.

    Int32 (TransactionId)

    :   Xid of the transaction.

    Int8

    :   A value of 1 indicates this is the first stream segment for this
        XID, 0 for any other stream segment.

Stream Stop

:   

    Byte1(\'E\')

    :   Identifies the message as a stream stop message.

Stream Commit

:   

    Byte1(\'c\')

    :   Identifies the message as a stream commit message.

    Int32 (TransactionId)

    :   Xid of the transaction.

    Int8(0)

    :   Flags; currently unused.

    Int64 (XLogRecPtr)

    :   The LSN of the commit.

    Int64 (XLogRecPtr)

    :   The end LSN of the transaction.

    Int64 (TimestampTz)

    :   Commit timestamp of the transaction. The value is in number of
        microseconds since PostgreSQL epoch (2000-01-01).

Stream Abort

:   

    Byte1(\'A\')

    :   Identifies the message as a stream abort message.

    Int32 (TransactionId)

    :   Xid of the transaction.

    Int32 (TransactionId)

    :   Xid of the subtransaction (will be same as xid of the
        transaction for top-level transactions).

The following messages (Begin Prepare, Prepare, Commit Prepared,
Rollback Prepared, Stream Prepare) are available since protocol version
3.

Begin Prepare

:   

    Byte1(\'b\')

    :   Identifies the message as the beginning of a two-phase
        transaction message.

    Int64 (XLogRecPtr)

    :   The LSN of the prepare.

    Int64 (XLogRecPtr)

    :   The end LSN of the prepared transaction.

    Int64 (TimestampTz)

    :   Prepare timestamp of the transaction. The value is in number of
        microseconds since PostgreSQL epoch (2000-01-01).

    Int32 (TransactionId)

    :   Xid of the transaction.

    String

    :   The user defined GID of the two-phase transaction.

Prepare

:   

    Byte1(\'P\')

    :   Identifies the message as a two-phase prepared transaction
        message.

    Int8(0)

    :   Flags; currently unused.

    Int64 (XLogRecPtr)

    :   The LSN of the prepare.

    Int64 (XLogRecPtr)

    :   The end LSN of the prepared transaction.

    Int64 (TimestampTz)

    :   Prepare timestamp of the transaction. The value is in number of
        microseconds since PostgreSQL epoch (2000-01-01).

    Int32 (TransactionId)

    :   Xid of the transaction.

    String

    :   The user defined GID of the two-phase transaction.

Commit Prepared

:   

    Byte1(\'K\')

    :   Identifies the message as the commit of a two-phase transaction
        message.

    Int8(0)

    :   Flags; currently unused.

    Int64 (XLogRecPtr)

    :   The LSN of the commit prepared.

    Int64 (XLogRecPtr)

    :   The end LSN of the commit prepared transaction.

    Int64 (TimestampTz)

    :   Commit timestamp of the transaction. The value is in number of
        microseconds since PostgreSQL epoch (2000-01-01).

    Int32 (TransactionId)

    :   Xid of the transaction.

    String

    :   The user defined GID of the two-phase transaction.

Rollback Prepared

:   

    Byte1(\'r\')

    :   Identifies the message as the rollback of a two-phase
        transaction message.

    Int8(0)

    :   Flags; currently unused.

    Int64 (XLogRecPtr)

    :   The end LSN of the prepared transaction.

    Int64 (XLogRecPtr)

    :   The end LSN of the rollback prepared transaction.

    Int64 (TimestampTz)

    :   Prepare timestamp of the transaction. The value is in number of
        microseconds since PostgreSQL epoch (2000-01-01).

    Int64 (TimestampTz)

    :   Rollback timestamp of the transaction. The value is in number of
        microseconds since PostgreSQL epoch (2000-01-01).

    Int32 (TransactionId)

    :   Xid of the transaction.

    String

    :   The user defined GID of the two-phase transaction.

Stream Prepare

:   

    Byte1(\'p\')

    :   Identifies the message as a two-phase stream prepare message.

    Int8(0)

    :   Flags; currently unused.

    Int64 (XLogRecPtr)

    :   The LSN of the prepare.

    Int64 (XLogRecPtr)

    :   The end LSN of the prepare transaction.

    Int64 (TimestampTz)

    :   Prepare timestamp of the transaction. The value is in number of
        microseconds since PostgreSQL epoch (2000-01-01).

    Int32 (TransactionId)

    :   Xid of the transaction.

    String

    :   The user defined GID of the two-phase transaction.

The following message parts are shared by the above messages.

TupleData

:   

    Int16

    :   Number of columns.

    Next, one of the following submessages appears for each column
    (except generated columns):

    Byte1(\'n\')

    :   Identifies the data as NULL value.

    Or

    Byte1(\'u\')

    :   Identifies unchanged TOASTed value (the actual value is not
        sent).

    Or

    Byte1(\'t\')

    :   Identifies the data as text formatted value.

    Or

    Byte1(\'b\')

    :   Identifies the data as binary formatted value.

    Int32

    :   Length of the column value.

    Byten

    :   The value of the column, either in binary or in text format. (As
        specified in the preceding format byte). n is the above length.

Summary of Changes since Protocol 2.0 {#protocol-changes}
=====================================

This section provides a quick checklist of changes, for the benefit of
developers trying to update existing client libraries to protocol 3.0.

The initial startup packet uses a flexible list-of-strings format
instead of a fixed format. Notice that session default values for
run-time parameters can now be specified directly in the startup packet.
(Actually, you could do that before using the `options` field, but given
the limited width of `options` and the lack of any way to quote
whitespace in the values, it wasn\'t a very safe technique.)

All messages now have a length count immediately following the message
type byte (except for startup packets, which have no type byte). Also
note that PasswordMessage now has a type byte.

ErrorResponse and NoticeResponse (\'`E`\' and \'`N`\') messages now
contain multiple fields, from which the client code can assemble an
error message of the desired level of verbosity. Note that individual
fields will typically not end with a newline, whereas the single string
sent in the older protocol always did.

The ReadyForQuery (\'`Z`\') message includes a transaction status
indicator.

The distinction between BinaryRow and DataRow message types is gone; the
single DataRow message type serves for returning data in all formats.
Note that the layout of DataRow has changed to make it easier to parse.
Also, the representation of binary values has changed: it is no longer
directly tied to the server\'s internal representation.

There is a new "extended query" sub-protocol, which adds the frontend
message types Parse, Bind, Execute, Describe, Close, Flush, and Sync,
and the backend message types ParseComplete, BindComplete,
PortalSuspended, ParameterDescription, NoData, and CloseComplete.
Existing clients do not have to concern themselves with this
sub-protocol, but making use of it might allow improvements in
performance or functionality.

`COPY` data is now encapsulated into CopyData and CopyDone messages.
There is a well-defined way to recover from errors during `COPY`. The
special "`\.`" last line is not needed anymore, and is not sent during
`COPY OUT`. (It is still recognized as a terminator during `COPY IN`,
but its use is deprecated and will eventually be removed.) Binary `COPY`
is supported. The CopyInResponse and CopyOutResponse messages include
fields indicating the number of columns and the format of each column.

The layout of FunctionCall and FunctionCallResponse messages has
changed. FunctionCall can now support passing NULL arguments to
functions. It also can handle passing parameters and retrieving results
in either text or binary format. There is no longer any reason to
consider FunctionCall a potential security hole, since it does not offer
direct access to internal server data representations.

The backend sends ParameterStatus (\'`S`\') messages during connection
startup for all parameters it considers interesting to the client
library. Subsequently, a ParameterStatus message is sent whenever the
active value changes for any of these parameters.

The RowDescription (\'`T`\') message carries new table OID and column
number fields for each column of the described row. It also shows the
format code for each column.

The CursorResponse (\'`P`\') message is no longer generated by the
backend.

The NotificationResponse (\'`A`\') message has an additional string
field, which can carry a "payload" string passed from the `NOTIFY` event
sender.

The EmptyQueryResponse (\'`I`\') message used to include an empty string
parameter; this has been removed.
