第 50 章 Writing A Foreign Data Wrapper
目录
All operations on a foreign table are handled through its foreign data wrapper, which consists of a set of functions that the core server calls. 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 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.
注意
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.
50.1. Foreign Data Wrapper Functions #
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 第 35.9 节.
The handler function simply returns a struct of function pointers to
callback functions that will be called by the planner, executor, and
various maintenance commands.
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 callback functions are described in
第 50.2 节.
The validator function is responsible for validating options given in
CREATE and ALTER commands for its
foreign data wrapper, as well as foreign servers, user mappings, and
foreign tables using the wrapper.
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, either
ForeignDataWrapperRelationId,
ForeignServerRelationId,
UserMappingRelationId,
or ForeignTableRelationId).
If no validator function is supplied, options are not checked at object
creation time or object alteration time.