foreign data wrapper handler for 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 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 reference page also has some useful details. 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. 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 . 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 callback functions are described in . 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. The FDW handler function returns a palloc'd FdwRoutine struct containing pointers to the following callback functions: void PlanForeignScan (Oid foreigntableid, PlannerInfo *root, RelOptInfo *baserel); Create possible access paths for a scan on a foreign table. This is called when a query is planned. foreigntableid is the pg_class OID of the foreign table. root is the planner's global information about the query, and baserel is the planner's information about this table. The function must generate at least one access path (ForeignPath node) for a scan on the foreign table and must call add_path to add the path to baserel->pathlist. It's recommended to use create_foreignscan_path to build the ForeignPath node. The function may generate multiple access paths, e.g., a path which has valid pathkeys to represent a pre-sorted result. Each access path must contain cost estimates, and can contain any FDW-private information that is needed to execute the foreign scan at a later time. (Note that the private information must be represented in a form that copyObject knows how to copy.) The information in root and baserel can be used to reduce the amount of information that has to be fetched from the foreign table (and therefore reduce the cost estimate). baserel->baserestrictinfo is particularly interesting, as it contains restriction quals (WHERE clauses) that can be used to filter the rows to be fetched. (The FDW is not required to enforce these quals, as the finished plan will recheck them anyway.) baserel->reltargetlist can be used to determine which columns need to be fetched. In addition to returning cost estimates, the function should update baserel->rows to be the expected number of rows returned by the scan, after accounting for the filtering done by the restriction quals. The initial value of baserel->rows is just a constant default estimate, which should be replaced if at all possible. The function may also choose to update baserel->width if it can compute a better estimate of the average result row width. void ExplainForeignScan (ForeignScanState *node, ExplainState *es); Print additional EXPLAIN output for a foreign table scan. This can just return if there is no need to print anything. Otherwise, it should call ExplainPropertyText and related functions to add fields to the EXPLAIN output. The flag fields in es can be used to determine what to print, and the state of the ForeignScanState node can be inspected to provide runtime statistics in the EXPLAIN ANALYZE case. void BeginForeignScan (ForeignScanState *node, int eflags); Begin executing a foreign scan. This is called during executor startup. It should perform any initialization needed before the scan can start, but not start executing the actual scan (that should be done upon the first call to IterateForeignScan). The ForeignScanState node has already been created, but its fdw_state field is still NULL. Information about the table to scan is accessible through the ForeignScanState node (in particular, from the underlying ForeignScan plan node, which contains any FDW-private information provided by PlanForeignScan). Note that when (eflags & EXEC_FLAG_EXPLAIN_ONLY) is true, this function should not perform any externally-visible actions; it should only do the minimum required to make the node state valid for ExplainForeignScan and EndForeignScan. TupleTableSlot * IterateForeignScan (ForeignScanState *node); Fetch one row from the foreign source, returning it in a tuple table slot (the node's ScanTupleSlot should be used for this purpose). Return NULL if no more rows are available. The tuple table slot infrastructure allows either a physical or virtual tuple to be returned; in most cases the latter choice is preferable from a performance standpoint. Note that this is called in a short-lived memory context that will be reset between invocations. Create a memory context in BeginForeignScan if you need longer-lived storage, or use the es_query_cxt of the node's EState. The rows returned must match the column signature of the foreign table being scanned. If you choose to optimize away fetching columns that are not needed, you should insert nulls in those column positions. Note that PostgreSQL's executor doesn't care whether the rows returned violate the NOT NULL constraints which were defined on the foreign table columns - but the planner does care, and may optimize queries incorrectly if NULL values are present in a column declared not to contain them. If a NULL value is encountered when the user has declared that none should be present, it may be appropriate to raise an error (just as you would need to do in the case of a data type mismatch). void ReScanForeignScan (ForeignScanState *node); Restart the scan from the beginning. Note that any parameters the scan depends on may have changed value, so the new scan does not necessarily return exactly the same rows. void EndForeignScan (ForeignScanState *node); End the scan and release resources. It is normally not important to release palloc'd memory, but for example open files and connections to remote servers should be cleaned up. The FdwRoutine struct type is declared in src/include/foreign/fdwapi.h, which see for additional details. Several helper functions are exported from the core server so that authors of foreign data wrappers can get easy access to attributes of FDW-related objects, such as FDW options. To use any of these functions, you need to include the header file foreign/foreign.h in your source file. That header also defines the struct types that are returned by these functions. ForeignDataWrapper * GetForeignDataWrapper(Oid fdwid); This function returns a ForeignDataWrapper object for the foreign-data wrapper with the given OID. A ForeignDataWrapper object contains properties of the FDW (see foreign/foreign.h for details). ForeignServer * GetForeignServer(Oid serverid); This function returns a ForeignServer object for the foreign server with the given OID. A ForeignServer object contains properties of the server (see foreign/foreign.h for details). UserMapping * GetUserMapping(Oid userid, Oid serverid); This function returns a UserMapping object for the user mapping of the given role on the given server. (If there is no mapping for the specific user, it will return the mapping for PUBLIC, or throw error if there is none.) A UserMapping object contains properties of the user mapping (see foreign/foreign.h for details). ForeignTable * GetForeignTable(Oid relid); This function returns a ForeignTable object for the foreign table with the given OID. A ForeignTable object contains properties of the foreign table (see foreign/foreign.h for details). List * GetForeignTableColumnOptions(Oid relid, AttrNumber attnum); This function returns the per-column FDW options for the column with the given foreign table OID and attribute number, in the form of a list of DefElem. NIL is returned if the column has no options. Some object types have name-based lookup functions in addition to the OID-based ones: ForeignDataWrapper * GetForeignDataWrapperByName(const char *name, bool missing_ok); This function returns a ForeignDataWrapper object for the foreign-data wrapper with the given name. If the wrapper is not found, return NULL if missing_ok is true, otherwise raise an error. ForeignServer * GetForeignServerByName(const char *name, bool missing_ok); This function returns a ForeignServer object for the foreign server with the given name. If the server is not found, return NULL if missing_ok is true, otherwise raise an error.