CREATE FOREIGN TABLE(7) PostgreSQL 15.7 Documentation CREATE FOREIGN TABLE(7)
NAME
CREATE_FOREIGN_TABLE - define a new foreign table
SYNOPSIS
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 }
[, ... ]
) ]
{ FOR VALUES partition_bound_spec | DEFAULT }
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 ]
and partition_bound_spec is:
IN ( partition_bound_expr [, ...] ) |
FROM ( { partition_bound_expr | MINVALUE | MAXVALUE } [, ...] )
TO ( { partition_bound_expr | MINVALUE | MAXVALUE } [, ...] ) |
WITH ( MODULUS numeric_literal, REMAINDER numeric_literal )
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 Chapter 8.
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 for more details.
PARTITION OF parent_table { FOR VALUES partition_bound_spec | DEFAULT }
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 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.)
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
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 CREATE SERVER
(CREATE_SERVER(7)).
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
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
The CREATE FOREIGN TABLE command largely conforms to the SQL standard;
however, much as with CREATE TABLE, 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
ALTER FOREIGN TABLE (ALTER_FOREIGN_TABLE(7)), DROP FOREIGN TABLE
(DROP_FOREIGN_TABLE(7)), CREATE TABLE (CREATE_TABLE(7)), CREATE SERVER
(CREATE_SERVER(7)), IMPORT FOREIGN SCHEMA (IMPORT_FOREIGN_SCHEMA(7))
PostgreSQL 15.7 2024 CREATE FOREIGN TABLE(7)
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