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 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 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, 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 . 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 GetForeignRelSize (PlannerInfo *root, RelOptInfo *baserel, Oid foreigntableid); Obtain relation size estimates for a foreign table. This is called at the beginning of planning for a query involving a foreign table. root is the planner's global information about the query; baserel is the planner's information about this table; and foreigntableid is the pg_class OID of the foreign table. (foreigntableid could be obtained from the planner data structures, but it's passed explicitly to save effort.) This function should update baserel->rows to be the expected number of rows returned by the table 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. See for additional information. void GetForeignPaths (PlannerInfo *root, RelOptInfo *baserel, Oid foreigntableid); Create possible access paths for a scan on a foreign table. This is called during query planning. The parameters are the same as for GetForeignRelSize, which has already been called. This function must generate at least one access path (ForeignPath node) for a scan on the foreign table and must call add_path to add each such path to baserel->pathlist. It's recommended to use create_foreignscan_path to build the ForeignPath nodes. The function can 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 identify the specific scan method intended. See for additional information. ForeignScan * GetForeignPlan (PlannerInfo *root, RelOptInfo *baserel, Oid foreigntableid, ForeignPath *best_path, List *tlist, List *scan_clauses); Create a ForeignScan plan node from the selected foreign access path. This is called at the end of query planning. The parameters are as for GetForeignRelSize, plus the selected ForeignPath (previously produced by GetForeignPaths), the target list to be emitted by the plan node, and the restriction clauses to be enforced by the plan node. This function must create and return a ForeignScan plan node; it's recommended to use make_foreignscan to build the ForeignScan node. See for additional information. 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 run-time 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 GetForeignPlan). 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. bool AnalyzeForeignTable (Relation relation, AcquireSampleRowsFunc *func, BlockNumber *totalpages); This function is called when is executed on a foreign table. If the FDW can collect statistics for this foreign table, it should return true, and provide a pointer to a function that will collect sample rows from the table in func, plus the estimated size of the table in pages in totalpages. Otherwise, return false. If the FDW does not support collecting statistics for any tables, the AnalyzeForeignTable pointer can be set to NULL. If provided, the sample collection function must have the signature int AcquireSampleRowsFunc (Relation relation, int elevel, HeapTuple *rows, int targrows, double *totalrows, double *totaldeadrows); A random sample of up to targrows rows should be collected from the table and stored into the caller-provided rows array. The actual number of rows collected must be returned. In addition, store estimates of the total numbers of live and dead rows in the table into the output parameters totalrows and totaldeadrows. (Set totaldeadrows to zero if the FDW does not have any concept of dead rows.) 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. The FDW callback functions GetForeignRelSize, GetForeignPaths, and GetForeignPlan must fit into the workings of the PostgreSQL planner. Here are some notes about what they must do. 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). baserel->baserestrictinfo is particularly interesting, as it contains restriction quals (WHERE clauses) that should be used to filter the rows to be fetched. (The FDW itself is not required to enforce these quals, as the core executor can check them instead.) baserel->reltargetlist can be used to determine which columns need to be fetched; but note that it only lists columns that have to be emitted by the ForeignScan plan node, not columns that are used in qual evaluation but not output by the query. Various private fields are available for the FDW planning functions to keep information in. Generally, whatever you store in FDW private fields should be palloc'd, so that it will be reclaimed at the end of planning. baserel->fdw_private is a void pointer that is available for FDW planning functions to store information relevant to the particular foreign table. The core planner does not touch it except to initialize it to NULL when the baserel node is created. It is useful for passing information forward from GetForeignRelSize to GetForeignPaths and/or GetForeignPaths to GetForeignPlan, thereby avoiding recalculation. GetForeignPaths can identify the meaning of different access paths by storing private information in the fdw_private field of ForeignPath nodes. fdw_private is declared as a List pointer, but could actually contain anything since the core planner does not touch it. However, best practice is to use a representation that's dumpable by nodeToString, for use with debugging support available in the backend. GetForeignPlan can examine the fdw_private field of the selected ForeignPath node, and can generate fdw_exprs and fdw_private lists to be placed in the ForeignScan plan node, where they will be available at execution time. Both of these lists must be represented in a form that copyObject knows how to copy. The fdw_private list has no other restrictions and is not interpreted by the core backend in any way. The fdw_exprs list, if not NIL, is expected to contain expression trees that are intended to be executed at run time. These trees will undergo post-processing by the planner to make them fully executable. In GetForeignPlan, generally the passed-in target list can be copied into the plan node as-is. The passed scan_clauses list contains the same clauses as baserel->baserestrictinfo, but may be re-ordered for better execution efficiency. In simple cases the FDW can just strip RestrictInfo nodes from the scan_clauses list (using extract_actual_clauses) and put all the clauses into the plan node's qual list, which means that all the clauses will be checked by the executor at run time. More complex FDWs may be able to check some of the clauses internally, in which case those clauses can be removed from the plan node's qual list so that the executor doesn't waste time rechecking them. As an example, the FDW might identify some restriction clauses of the form foreign_variable = sub_expression, which it determines can be executed on the remote server given the locally-evaluated value of the sub_expression. The actual identification of such a clause should happen during GetForeignPaths, since it would affect the cost estimate for the path. The path's fdw_private field would probably include a pointer to the identified clause's RestrictInfo node. Then GetForeignPlan would remove that clause from scan_clauses, but add the sub_expression to fdw_exprs to ensure that it gets massaged into executable form. It would probably also put control information into the plan node's fdw_private field to tell the execution functions what to do at run time. The query transmitted to the remote server would involve something like WHERE foreign_variable = $1, with the parameter value obtained at run time from evaluation of the fdw_exprs expression tree. The FDW should always construct at least one path that depends only on the table's restriction clauses. In join queries, it might also choose to construct path(s) that depend on join clauses, for example foreign_variable = local_variable. Such clauses will not be found in baserel->baserestrictinfo but must be sought in the relation's join lists. A path using such a clause is called a parameterized path. It must show the other relation(s) as required_outer and list the specific join clause(s) in param_clauses. In GetForeignPlan, the local_variable portion of the join clause would be added to fdw_exprs, and then at run time the case works the same as for an ordinary restriction clause.