PostgreSQL 9.1beta2 Documentation
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# Chapter 50. Writing A Foreign Data Wrapper

All operations on a foreign table are handled through its foreign data wrapper, which consists of a set of functions that the planner and executor call. The foreign data wrapper is responsible for fetching data from the remote data source and returning it to the PostgreSQL executor. This chapter outlines how to write a new foreign data wrapper.

The FDW author needs to implement a handler function, and optionally a validator function. Both functions must be written in a compiled language such as C, using the version-1 interface. For details on C language calling conventions and dynamic loading, see Section 35.9.

The handler function simply returns a struct of function pointers to callback functions that will be called by the planner and executor. Most of the effort in writing an FDW is in implementing these callback functions. The handler function must be registered with PostgreSQL as taking no arguments and returning the special pseudo-type `fdw_handler`. The callback functions are plain C functions and are not visible or callable at the SQL level.

The validator function is responsible for validating options given in the `CREATE FOREIGN DATA WRAPPER`, `CREATE SERVER` and `CREATE FOREIGN TABLE` commands. The validator function must be registered as taking two arguments, a text array containing the options to be validated, and an OID representing the type of object the options are associated with (in the form of the OID of the system catalog the object would be stored in). If no validator function is supplied, the options are not checked at object creation time.

The foreign data wrappers included in the standard distribution are good references when trying to write your own. Look into the `contrib/file_fdw` subdirectory of the source tree. The CREATE FOREIGN DATA WRAPPER reference page also has some useful details.

> Note: The SQL standard specifies an interface for writing foreign data wrappers. However, PostgreSQL does not implement that API, because the effort to accommodate it into PostgreSQL would be large, and the standard API hasn't gained wide adoption anyway.

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