DECLARE

cursor

DECLARE

DECLARE

7

SQL - Language Statements

DECLARE

define a cursor

DECLARE

name

\[ BINARY \] \[ ASENSITIVE \| INSENSITIVE \] \[ \[ NO \] SCROLL \]
CURSOR \[ { WITH \| WITHOUT } HOLD \] FOR

query

Description
===========

`DECLARE` allows a user to create cursors, which can be used to retrieve
a small number of rows at a time out of a larger query. After the cursor
is created, rows are fetched from it using [`FETCH`](#sql-fetch).

> **Note**
>
> This page describes usage of cursors at the SQL command level. If you
> are trying to use cursors inside a PL/pgSQL function, the rules are
> different MDASH see [???](#plpgsql-cursors).

Parameters
==========

name

:   The name of the cursor to be created.

`BINARY`

:   Causes the cursor to return data in binary rather than in text
    format.

`ASENSITIVE`; `INSENSITIVE`

:   Cursor sensitivity determines whether changes to the data underlying
    the cursor, done in the same transaction, after the cursor has been
    declared, are visible in the cursor. `INSENSITIVE` means they are
    not visible, `ASENSITIVE` means the behavior is
    implementation-dependent. A third behavior, `SENSITIVE`, meaning
    that such changes are visible in the cursor, is not available in
    PostgreSQL. In PostgreSQL, all cursors are insensitive; so these key
    words have no effect and are only accepted for compatibility with
    the SQL standard.

    Specifying `INSENSITIVE` together with `FOR
          UPDATE` or `FOR SHARE` is an error.

`SCROLL`; `NO SCROLL`

:   `SCROLL` specifies that the cursor can be used to retrieve rows in a
    nonsequential fashion (e.g., backward). Depending upon the
    complexity of the query\'s execution plan, specifying `SCROLL` might
    impose a performance penalty on the query\'s execution time.
    `NO SCROLL` specifies that the cursor cannot be used to retrieve
    rows in a nonsequential fashion. The default is to allow scrolling
    in some cases; this is not the same as specifying `SCROLL`. See
    [Notes](#sql-declare-notes) below for details.

`WITH HOLD`; `WITHOUT HOLD`

:   `WITH HOLD` specifies that the cursor can continue to be used after
    the transaction that created it successfully commits. `WITHOUT HOLD`
    specifies that the cursor cannot be used outside of the transaction
    that created it. If neither `WITHOUT HOLD` nor `WITH HOLD` is
    specified, `WITHOUT
          HOLD` is the default.

query

:   A [`SELECT`](#sql-select) or [`VALUES`](#sql-values) command which
    will provide the rows to be returned by the cursor.

The key words `ASENSITIVE`, `BINARY`, `INSENSITIVE`, and `SCROLL` can
appear in any order.

Notes {#sql-declare-notes}
=====

Normal cursors return data in text format, the same as a `SELECT` would
produce. The `BINARY` option specifies that the cursor should return
data in binary format. This reduces conversion effort for both the
server and client, at the cost of more programmer effort to deal with
platform-dependent binary data formats. As an example, if a query
returns a value of one from an integer column, you would get a string of
`1` with a default cursor, whereas with a binary cursor you would get a
4-byte field containing the internal representation of the value (in
big-endian byte order).

Binary cursors should be used carefully. Many applications, including
psql, are not prepared to handle binary cursors and expect data to come
back in the text format.

> **Note**
>
> When the client application uses the "extended query" protocol to
> issue a `FETCH` command, the Bind protocol message specifies whether
> data is to be retrieved in text or binary format. This choice
> overrides the way that the cursor is defined. The concept of a binary
> cursor as such is thus obsolete when using extended query protocol
> MDASH any cursor can be treated as either text or binary.

Unless `WITH HOLD` is specified, the cursor created by this command can
only be used within the current transaction. Thus, `DECLARE` without
`WITH
    HOLD` is useless outside a transaction block: the cursor would
survive only to the completion of the statement. Therefore PostgreSQL
reports an error if such a command is used outside a transaction block.
Use [`BEGIN`](#sql-begin) and [`COMMIT`](#sql-commit) (or
[`ROLLBACK`](#sql-rollback)) to define a transaction block.

If `WITH HOLD` is specified and the transaction that created the cursor
successfully commits, the cursor can continue to be accessed by
subsequent transactions in the same session. (But if the creating
transaction is aborted, the cursor is removed.) A cursor created with
`WITH HOLD` is closed when an explicit `CLOSE` command is issued on it,
or the session ends. In the current implementation, the rows represented
by a held cursor are copied into a temporary file or memory area so that
they remain available for subsequent transactions.

`WITH HOLD` may not be specified when the query includes `FOR UPDATE` or
`FOR SHARE`.

The `SCROLL` option should be specified when defining a cursor that will
be used to fetch backwards. This is required by the SQL standard.
However, for compatibility with earlier versions, PostgreSQL will allow
backward fetches without `SCROLL`, if the cursor\'s query plan is simple
enough that no extra overhead is needed to support it. However,
application developers are advised not to rely on using backward fetches
from a cursor that has not been created with `SCROLL`. If `NO SCROLL` is
specified, then backward fetches are disallowed in any case.

Backward fetches are also disallowed when the query includes
`FOR UPDATE` or `FOR SHARE`; therefore `SCROLL` may not be specified in
this case.

> **Caution**
>
> Scrollable cursors may give unexpected results if they invoke any
> volatile functions (see [???](#xfunc-volatility)). When a previously
> fetched row is re-fetched, the functions might be re-executed, perhaps
> leading to results different from the first time. It\'s best to
> specify `NO SCROLL` for a query involving volatile functions. If that
> is not practical, one workaround is to declare the cursor
> `SCROLL WITH HOLD` and commit the transaction before reading any rows
> from it. This will force the entire output of the cursor to be
> materialized in temporary storage, so that volatile functions are
> executed exactly once for each row.

If the cursor\'s query includes `FOR UPDATE` or `FOR
    SHARE`, then returned rows are locked at the time they are first
fetched, in the same way as for a regular [`SELECT`](#sql-select)
command with these options. In addition, the returned rows will be the
most up-to-date versions.

> **Caution**
>
> It is generally recommended to use `FOR UPDATE` if the cursor is
> intended to be used with `UPDATE ... WHERE CURRENT OF` or
> `DELETE ... WHERE CURRENT OF`. Using `FOR UPDATE` prevents other
> sessions from changing the rows between the time they are fetched and
> the time they are updated. Without `FOR UPDATE`, a subsequent
> `WHERE CURRENT OF` command will have no effect if the row was changed
> since the cursor was created.
>
> Another reason to use `FOR UPDATE` is that without it, a subsequent
> `WHERE CURRENT OF` might fail if the cursor query does not meet the
> SQL standard\'s rules for being "simply updatable" (in particular, the
> cursor must reference just one table and not use grouping or
> `ORDER BY`). Cursors that are not simply updatable might work, or
> might not, depending on plan choice details; so in the worst case, an
> application might work in testing and then fail in production. If
> `FOR UPDATE` is specified, the cursor is guaranteed to be updatable.
>
> The main reason not to use `FOR UPDATE` with `WHERE
>      CURRENT OF` is if you need the cursor to be scrollable, or to be
> isolated from concurrent updates (that is, continue to show the old
> data). If this is a requirement, pay close heed to the caveats shown
> above.

The SQL standard only makes provisions for cursors in embedded SQL. The
PostgreSQL server does not implement an `OPEN` statement for cursors; a
cursor is considered to be open when it is declared. However, ECPG, the
embedded SQL preprocessor for PostgreSQL, supports the standard SQL
cursor conventions, including those involving `DECLARE` and `OPEN`
statements.

You can see all available cursors by querying the
[pg\_cursors](#view-pg-cursors) system view.

Examples
========

To declare a cursor:

    DECLARE liahona CURSOR FOR SELECT * FROM films;

See [???](#sql-fetch) for more examples of cursor usage.

Compatibility
=============

The SQL standard allows cursors only in embedded SQL and in modules.
PostgreSQL permits cursors to be used interactively.

According to the SQL standard, changes made to insensitive cursors by
`UPDATE ... WHERE CURRENT OF` and `DELETE
   ... WHERE CURRENT OF` statements are visible in that same cursor.
PostgreSQL treats these statements like all other data changing
statements in that they are not visible in insensitive cursors.

Binary cursors are a PostgreSQL extension.

See Also
========
