CREATE FOREIGN DATA WRAPPER
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## CREATE FOREIGN DATA WRAPPER

CREATE FOREIGN DATA WRAPPER — define a new foreign-data wrapper

## Synopsis

    CREATE FOREIGN DATA WRAPPER name
        [ HANDLER handler_function | NO HANDLER ]
        [ VALIDATOR validator_function | NO VALIDATOR ]
        [ CONNECTION connection_function | NO CONNECTION ]
        [ OPTIONS ( option 'value' [, ... ] ) ]

## Description

`CREATE FOREIGN DATA WRAPPER` creates a new foreign-data wrapper. The user who defines a foreign-data wrapper becomes its owner.

The foreign-data wrapper name must be unique within the database.

Only superusers can create foreign-data wrappers.

## Parameters

`name`

  The name of the foreign-data wrapper to be created.

`HANDLER `handler_function``

  `handler_function` is the name of a previously registered function that will be called to retrieve the execution functions for foreign tables. The handler function must take no arguments, and its return type must be `fdw_handler`.

  It is possible to create a foreign-data wrapper with no handler function, but foreign tables using such a wrapper can only be declared, not accessed.

`VALIDATOR `validator_function``

  `validator_function` is the name of a previously registered function that will be called to check the generic options given to the foreign-data wrapper, as well as options for foreign servers, user mappings and foreign tables using the foreign-data wrapper. If no validator function or `NO VALIDATOR` is specified, then options will not be checked at creation time. (Foreign-data wrappers will possibly ignore or reject invalid option specifications at run time, depending on the implementation.) The validator function must take two arguments: one of type `text[]`, which will contain the array of options as stored in the system catalogs, and one of type `oid`, which will be the OID of the system catalog containing the options. The return type is ignored; the function should report invalid options using the `ereport(ERROR)` function.

`CONNECTION `connection_function``

  `connection_function` is the name of a previously registered function that will be called to generate the postgres connection string when a foreign server is used as part of CREATE SUBSCRIPTION. If no connection function or `NO CONNECTION` is specified, then servers using this foreign data wrapper cannot be used for `CREATE SUBSCRIPTION`. The connection function must take three arguments: one of type `oid` for the user, one of type `oid` for the server, and an unused third argument of type `internal` (which prevents calling the function in other contexts).

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

  This clause specifies options for the new foreign-data wrapper. The allowed option names and values are specific to each foreign data wrapper and are validated using the foreign-data wrapper's validator function. Option names must be unique.

## Notes

PostgreSQL's foreign-data functionality is still under active development. Optimization of queries is primitive (and mostly left to the wrapper, too). Thus, there is considerable room for future performance improvements.

## Examples

Create a useless foreign-data wrapper `dummy`:

    CREATE FOREIGN DATA WRAPPER dummy;

Create a foreign-data wrapper `file` with handler function `file_fdw_handler`:

    CREATE FOREIGN DATA WRAPPER file HANDLER file_fdw_handler;

Create a foreign-data wrapper `mywrapper` with some options:

    CREATE FOREIGN DATA WRAPPER mywrapper
        OPTIONS (debug 'true');

## Compatibility

`CREATE FOREIGN DATA WRAPPER` conforms to ISO/IEC 9075-9 (SQL/MED), with the exception that the `HANDLER` and `VALIDATOR` clauses are extensions and the standard clauses `LIBRARY` and `LANGUAGE` are not implemented in PostgreSQL.

Note, however, that the SQL/MED functionality as a whole is not yet conforming.

## See Also

ALTER FOREIGN DATA WRAPPER, DROP FOREIGN DATA WRAPPER, CREATE SERVER, CREATE USER MAPPING, CREATE FOREIGN TABLE

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