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CREATE FOREIGN TABLE

Defines a new foreign table.

Synopsis

CREATE FOREIGN TABLE [ IF NOT EXISTS ] <table_name> ( [
    <column_name> <data_type> [ OPTIONS ( <option> '<value>' [, ... ] ) ] [ COLLATE <collation> ] [ <column_constraint> [ ... ] ]
      [, ... ]
] )
    SERVER <server_name>
  [ OPTIONS ( [ mpp_execute { 'master' | 'any' | 'all segments' } [, ] ] <option> '<value>' [, ... ] ) ]

where column_constraint is:

[ CONSTRAINT <constraint_name> ]
{ NOT NULL |
  NULL |
  DEFAULT <default_expr> }

Description

CREATE FOREIGN TABLE creates a new foreign table in the current database. The user who creates the foreign table becomes its owner.

If you schema-qualify the table name (for example, CREATE FOREIGN TABLE myschema.mytable …​), Greengage DB creates the table in the specified schema. Otherwise, the foreign table is created in the current schema. The name of the foreign table must be distinct from the name of any other foreign table, table, sequence, index, or view in the same schema.

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

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

Parameters

Parameter Description

IF NOT EXISTS

Do not throw an error if a relation with the same name already exists. Greengage DB issues a notice 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 foreign table to create

column_name

The name of a column to create in the new foreign table

data_type

The data type of the column, including array specifiers

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

DEFAULT <default_expr>

The DEFAULT clause assigns a default value for the column whose definition it appears within. The value is any variable-free expression. Greengage DB does not allow subqueries and cross-references to other columns in the current table. The data type of the default expression must match the data type of the column. Greengage DB uses the default expression 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

server_name

The name of an existing server to use for the foreign table. For details on defining a server, see CREATE SERVER

OPTIONS ( <option> '<value>' [, …​ ] )

The options for the new foreign table or one of its columns. While option names must be unique, a table option and a column option may have the same name. The option names and values are foreign data wrapper-specific. Greengage DB validates the options and values using the foreign data wrapper’s <validator_function>

mpp_execute { 'master' | 'any' | 'all segments' }

A Greengage DB-specific option that identifies the host from which the foreign data wrapper reads or writes data:

  • master (the default) — reads or writes data from the master host.

  • any — reads data from either the master host or any one segment, depending on which path costs less.

  • all segments — reads or writes data from all segments. To support this option value, the foreign data wrapper must have a policy that matches the segments to data.

    NOTE

    Greengage DB supports parallel writes to foreign tables only when you set mpp_execute 'all segments'.

Support for the foreign table mpp_execute option, and the specific modes, is foreign data wrapper-specific. The mpp_execute option can be specified in multiple commands: CREATE FOREIGN TABLE, CREATE SERVER, and CREATE FOREIGN DATA WRAPPER. The foreign table setting takes precedence over the foreign server setting, followed by the foreign data wrapper setting

Notes

The GPORCA optimizer does not support foreign tables. A query on a foreign table always falls back to the Postgres planner.

Examples

Create a foreign table named films with the server named 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;

Compatibility

CREATE FOREIGN TABLE largely conforms to the SQL standard. However, much as with CREATE TABLE, Greengage DB permits NULL constraints and zero-column foreign tables. The ability to specify a default value is a Greengage DB extension, as is the mpp_execute option.

See also

Previous
CREATE FOREIGN DATA WRAPPER
Next
CREATE FUNCTION