CREATE FOREIGN TABLE

CREATE FOREIGN TABLE

7

SQL - Language Statements

CREATE FOREIGN TABLE

define a new foreign table

CREATE FOREIGN TABLE \[ IF NOT EXISTS \]

table\_name

( \[ {

column\_name

data\_type

\[ OPTIONS (

option

\'

value

\' \[, \... \] ) \] \[ COLLATE

collation

\] \[

column\_constraint

\[ \... \] \] \|

table\_constraint

} \[, \... \] \] ) \[ INHERITS (

parent\_table

\[, \... \] ) \] SERVER

server\_name

\[ OPTIONS (

option

\'

value

\' \[, \... \] ) \] CREATE FOREIGN TABLE \[ IF NOT EXISTS \]

table\_name

PARTITION OF

parent\_table

\[ ( {

column\_name

\[ WITH OPTIONS \] \[

column\_constraint

\[ \... \] \] \|

table\_constraint

} \[, \... \] ) \]

partition\_bound\_spec

SERVER

server\_name

\[ OPTIONS (

option

\'

value

\' \[, \... \] ) \]

where

column\_constraint

is:

\[ CONSTRAINT

constraint\_name

\] { NOT NULL \| NULL \| CHECK (

expression

) \[ NO INHERIT \] \| DEFAULT

default\_expr

\| GENERATED ALWAYS AS (

generation\_expr

) STORED }

and

table\_constraint

is:

\[ CONSTRAINT

constraint\_name

\] CHECK (

expression

) \[ NO INHERIT \]

Description {#sql-createforeigntable-description}
===========

`CREATE FOREIGN TABLE` creates a new foreign table in the current
database. The table will be owned by the user issuing the command.

If a schema name is given (for example, `CREATE FOREIGN TABLE
   myschema.mytable ...`) then the table is created in the specified
schema. Otherwise it is created in the current schema. The name of the
foreign table must be distinct from the name of any other relation
(table, sequence, index, view, materialized view, or foreign table) in
the same schema.

`CREATE FOREIGN TABLE` also automatically creates a data type that
represents the composite type corresponding to one row of the foreign
table. Therefore, foreign tables cannot have the same name as any
existing data type in the same schema.

If `PARTITION OF` clause is specified then the table is created as a
partition of `parent_table` with specified bounds.

To be able to create a foreign table, you must have `USAGE` privilege on
the foreign server, as well as `USAGE` privilege on all column types
used in the table.

Parameters
==========

`IF NOT EXISTS`

:   Do not throw an error if a relation with the same name already
    exists. A notice is issued in this case. Note that there is no
    guarantee that the existing relation is anything like the one that
    would have been created.

table\_name

:   The name (optionally schema-qualified) of the table to be created.

column\_name

:   The name of a column to be created in the new table.

data\_type

:   The data type of the column. This can include array specifiers. For
    more information on the data types supported by PostgreSQL, refer to
    [???](#datatype).

`COLLATE collation`

:   The `COLLATE` clause assigns a collation to the column (which must
    be of a collatable data type). If not specified, the column data
    type\'s default collation is used.

`INHERITS ( parent_table [, ... ] )`

:   The optional `INHERITS` clause specifies a list of tables from which
    the new foreign table automatically inherits all columns. Parent
    tables can be plain tables or foreign tables. See the similar form
    of [`CREATE TABLE`](#sql-createtable) for more details.

`PARTITION OF parent_table FOR VALUES partition_bound_spec`

:   This form can be used to create the foreign table as partition of
    the given parent table with specified partition bound values. See
    the similar form of [`CREATE TABLE`](#sql-createtable) for more
    details. Note that it is currently not allowed to create the foreign
    table as a partition of the parent table if there are `UNIQUE`
    indexes on the parent table. (See also
    [`ALTER TABLE ATTACH PARTITION`](#sql-altertable).)

`CONSTRAINT constraint_name`

:   An optional name for a column or table constraint. If the constraint
    is violated, the constraint name is present in error messages, so
    constraint names like `col must be positive` can be used to
    communicate helpful constraint information to client applications.
    (Double-quotes are needed to specify constraint names that contain
    spaces.) If a constraint name is not specified, the system generates
    a name.

`NOT NULL`

:   The column is not allowed to contain null values.

`NULL`

:   The column is allowed to contain null values. This is the default.

    This clause is only provided for compatibility with non-standard SQL
    databases. Its use is discouraged in new applications.

`CHECK ( expression ) [ NO INHERIT ] `

:   The `CHECK` clause specifies an expression producing a Boolean
    result which each row in the foreign table is expected to satisfy;
    that is, the expression should produce TRUE or UNKNOWN, never FALSE,
    for all rows in the foreign table. A check constraint specified as a
    column constraint should reference that column\'s value only, while
    an expression appearing in a table constraint can reference multiple
    columns.

    Currently, `CHECK` expressions cannot contain subqueries nor refer
    to variables other than columns of the current row. The system
    column `tableoid` may be referenced, but not any other system
    column.

    A constraint marked with `NO INHERIT` will not propagate to child
    tables.

`DEFAULT
    default_expr`

:   The `DEFAULT` clause assigns a default data value for the column
    whose column definition it appears within. The value is any
    variable-free expression (subqueries and cross-references to other
    columns in the current table are not allowed). The data type of the
    default expression must match the data type of the column.

    The default expression will be used in any insert operation that
    does not specify a value for the column. If there is no default for
    a column, then the default is null.

`GENERATED ALWAYS AS ( generation_expr ) STORED`generated column

:   This clause creates the column as a generated column. The column
    cannot be written to, and when read the result of the specified
    expression will be returned.

    The keyword `STORED` is required to signify that the column will be
    computed on write. (The computed value will be presented to the
    foreign-data wrapper for storage and must be returned on reading.)

    The generation expression can refer to other columns in the table,
    but not other generated columns. Any functions and operators used
    must be immutable. References to other tables are not allowed.

server\_name

:   The name of an existing foreign server to use for the foreign table.
    For details on defining a server, see [???](#sql-createserver).

`OPTIONS ( option 'value' [, ...] )`

:   Options to be associated with the new foreign table or one of its
    columns. The allowed option names and values are specific to each
    foreign data wrapper and are validated using the foreign-data
    wrapper\'s validator function. Duplicate option names are not
    allowed (although it\'s OK for a table option and a column option to
    have the same name).

Notes
=====

Constraints on foreign tables (such as `CHECK` or `NOT NULL` clauses)
are not enforced by the core PostgreSQL system, and most foreign data
wrappers do not attempt to enforce them either; that is, the constraint
is simply assumed to hold true. There would be little point in such
enforcement since it would only apply to rows inserted or updated via
the foreign table, and not to rows modified by other means, such as
directly on the remote server. Instead, a constraint attached to a
foreign table should represent a constraint that is being enforced by
the remote server.

Some special-purpose foreign data wrappers might be the only access
mechanism for the data they access, and in that case it might be
appropriate for the foreign data wrapper itself to perform constraint
enforcement. But you should not assume that a wrapper does that unless
its documentation says so.

Although PostgreSQL does not attempt to enforce constraints on foreign
tables, it does assume that they are correct for purposes of query
optimization. If there are rows visible in the foreign table that do not
satisfy a declared constraint, queries on the table might produce errors
or incorrect answers. It is the user\'s responsibility to ensure that
the constraint definition matches reality.

> **Caution**
>
> When a foreign table is used as a partition of a partitioned table,
> there is an implicit constraint that its contents must satisfy the
> partitioning rule. Again, it is the user\'s responsibility to ensure
> that that is true, which is best done by installing a matching
> constraint on the remote server.

Within a partitioned table containing foreign-table partitions, an
`UPDATE` that changes the partition key value can cause a row to be
moved from a local partition to a foreign-table partition, provided the
foreign data wrapper supports tuple routing. However it is not currently
possible to move a row from a foreign-table partition to another
partition. An `UPDATE` that would require doing that will fail due to
the partitioning constraint, assuming that that is properly enforced by
the remote server.

Similar considerations apply to generated columns. Stored generated
columns are computed on insert or update on the local PostgreSQL server
and handed to the foreign-data wrapper for writing out to the foreign
data store, but it is not enforced that a query of the foreign table
returns values for stored generated columns that are consistent with the
generation expression. Again, this might result in incorrect query
results.

Examples {#sql-createforeigntable-examples}
========

Create foreign table films, which will be accessed through the server
film\_server:

    CREATE FOREIGN TABLE films (
        code        char(5) NOT NULL,
        title       varchar(40) NOT NULL,
        did         integer NOT NULL,
        date_prod   date,
        kind        varchar(10),
        len         interval hour to minute
    )
    SERVER film_server;

Create foreign table measurement\_y2016m07, which will be accessed
through the server server\_07, as a partition of the range partitioned
table measurement:

    CREATE FOREIGN TABLE measurement_y2016m07
        PARTITION OF measurement FOR VALUES FROM ('2016-07-01') TO ('2016-08-01')
        SERVER server_07;

Compatibility {#sql-createforeigntable-compatibility}
=============

The `CREATE FOREIGN TABLE` command largely conforms to the SQL standard;
however, much as with [`CREATE TABLE`](#sql-createtable), `NULL`
constraints and zero-column foreign tables are permitted. The ability to
specify column default values is also a PostgreSQL extension. Table
inheritance, in the form defined by PostgreSQL, is nonstandard.

See Also
========
