59.2. Foreign Data Wrapper Callback Routines

59.2. Foreign Data Wrapper Callback Routines
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59.2.1. FDW Routines for Scanning Foreign Tables
59.2.2. FDW Routines for Scanning Foreign Joins
59.2.3. FDW Routines for Planning Post-Scan/Join Processing
59.2.4. FDW Routines for Updating Foreign Tables
59.2.5. FDW Routines for TRUNCATE
59.2.6. FDW Routines for Row Locking
59.2.7. FDW Routines for EXPLAIN
59.2.8. FDW Routines for ANALYZE
59.2.9. FDW Routines for IMPORT FOREIGN SCHEMA
59.2.10. FDW Routines for Parallel Execution
59.2.11. FDW Routines for Asynchronous Execution
59.2.12. FDW Routines for Reparameterization of Paths

+ The FDW handler function returns a palloc'd FdwRoutine + struct containing pointers to the callback functions described below. + The scan-related functions are required, the rest are optional. +

+ The FdwRoutine struct type is declared in + src/include/foreign/fdwapi.h, which see for additional + details. +

59.2.1. FDW Routines for Scanning Foreign Tables

+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 that scans 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. + (The initial value is based on column data types and on column + average-width values measured by the last ANALYZE.) + Also, this function may update baserel->tuples if + it can compute a better estimate of the foreign table's total row count. + (The initial value is + from pg_class.reltuples + which represents the total row count seen by the + last ANALYZE; it will be -1 if + no ANALYZE has been done on this foreign table.) +

+ See Section 59.4 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 Section 59.4 for additional information. +

+ForeignScan *
+GetForeignPlan(PlannerInfo *root,
+               RelOptInfo *baserel,
+               Oid foreigntableid,
+               ForeignPath *best_path,
+               List *tlist,
+               List *scan_clauses,
+               Plan *outer_plan);
+

+ + 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, GetForeignJoinPaths, + or GetForeignUpperPaths), + the target list to be emitted by the plan node, + the restriction clauses to be enforced by the plan node, + and the outer subplan of the ForeignScan, + which is used for rechecks performed by RecheckForeignScan. + (If the path is for a join rather than a base + relation, foreigntableid is InvalidOid.) +

+ This function must create and return a ForeignScan plan + node; it's recommended to use make_foreignscan to build the + ForeignScan node. +

+ See Section 59.4 for additional information. +

+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). + eflags contains flag bits describing the executor's + operating mode for this plan node. +

+ 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 fdw_scan_tlist target + list if one was supplied, otherwise they must match the row type 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, or else generate a fdw_scan_tlist list with + those columns omitted. +

+ Note that PostgreSQL's executor doesn't care + whether the rows returned violate any constraints that were defined on + the foreign table — but the planner does care, and may optimize + queries incorrectly if there are rows visible in the foreign table that + do not satisfy a declared constraint. If a constraint is violated when + the user has declared that the constraint should hold true, 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. +

59.2.2. FDW Routines for Scanning Foreign Joins

+ If an FDW supports performing foreign joins remotely (rather than + by fetching both tables' data and doing the join locally), it should + provide this callback function: +

+void
+GetForeignJoinPaths(PlannerInfo *root,
+                    RelOptInfo *joinrel,
+                    RelOptInfo *outerrel,
+                    RelOptInfo *innerrel,
+                    JoinType jointype,
+                    JoinPathExtraData *extra);
+

+ Create possible access paths for a join of two (or more) foreign tables + that all belong to the same foreign server. This optional + function is called during query planning. As + with GetForeignPaths, this function should + generate ForeignPath path(s) for the + supplied joinrel + (use create_foreign_join_path to build them), + and call add_path to add these + paths to the set of paths considered for the join. But unlike + GetForeignPaths, it is not necessary that this function + succeed in creating at least one path, since paths involving local + joining are always possible. +

+ Note that this function will be invoked repeatedly for the same join + relation, with different combinations of inner and outer relations; it is + the responsibility of the FDW to minimize duplicated work. +

+ If a ForeignPath path is chosen for the join, it will + represent the entire join process; paths generated for the component + tables and subsidiary joins will not be used. Subsequent processing of + the join path proceeds much as it does for a path scanning a single + foreign table. One difference is that the scanrelid of + the resulting ForeignScan plan node should be set to zero, + since there is no single relation that it represents; instead, + the fs_relids field of the ForeignScan + node represents the set of relations that were joined. (The latter field + is set up automatically by the core planner code, and need not be filled + by the FDW.) Another difference is that, because the column list for a + remote join cannot be found from the system catalogs, the FDW must + fill fdw_scan_tlist with an appropriate list + of TargetEntry nodes, representing the set of columns + it will supply at run time in the tuples it returns. +

+ See Section 59.4 for additional information. +

59.2.3. FDW Routines for Planning Post-Scan/Join Processing

+ If an FDW supports performing remote post-scan/join processing, such as + remote aggregation, it should provide this callback function: +

+void
+GetForeignUpperPaths(PlannerInfo *root,
+                     UpperRelationKind stage,
+                     RelOptInfo *input_rel,
+                     RelOptInfo *output_rel,
+                     void *extra);
+

+ Create possible access paths for upper relation processing, + which is the planner's term for all post-scan/join query processing, such + as aggregation, window functions, sorting, and table updates. This + optional function is called during query planning. Currently, it is + called only if all base relation(s) involved in the query belong to the + same FDW. This function should generate ForeignPath + path(s) for any post-scan/join processing that the FDW knows how to + perform remotely + (use create_foreign_upper_path to build them), + and call add_path to add these paths to + the indicated upper relation. As with GetForeignJoinPaths, + it is not necessary that this function succeed in creating any paths, + since paths involving local processing are always possible. +

+ The stage parameter identifies which post-scan/join step is + currently being considered. output_rel is the upper relation + that should receive paths representing computation of this step, + and input_rel is the relation representing the input to this + step. The extra parameter provides additional details, + currently, it is set only for UPPERREL_PARTIAL_GROUP_AGG + or UPPERREL_GROUP_AGG, in which case it points to a + GroupPathExtraData structure; + or for UPPERREL_FINAL, in which case it points to a + FinalPathExtraData structure. + (Note that ForeignPath paths added + to output_rel would typically not have any direct dependency + on paths of the input_rel, since their processing is expected + to be done externally. However, examining paths previously generated for + the previous processing step can be useful to avoid redundant planning + work.) +

+ See Section 59.4 for additional information. +

59.2.4. FDW Routines for Updating Foreign Tables

+ If an FDW supports writable foreign tables, it should provide + some or all of the following callback functions depending on + the needs and capabilities of the FDW: +

+void
+AddForeignUpdateTargets(PlannerInfo *root,
+                        Index rtindex,
+                        RangeTblEntry *target_rte,
+                        Relation target_relation);
+

+ + UPDATE and DELETE operations are performed + against rows previously fetched by the table-scanning functions. The + FDW may need extra information, such as a row ID or the values of + primary-key columns, to ensure that it can identify the exact row to + update or delete. To support that, this function can add extra hidden, + or “junk”, target columns to the list of columns that are to be + retrieved from the foreign table during an UPDATE or + DELETE. +

+ To do that, construct a Var representing + an extra value you need, and pass it + to add_row_identity_var, along with a name for + the junk column. (You can do this more than once if several columns + are needed.) You must choose a distinct junk column name for each + different Var you need, except + that Vars that are identical except for + the varno field can and should share a + column name. + The core system uses the junk column names + tableoid for a + table's tableoid column, + ctid + or ctidN + for ctid, + wholerow + for a whole-row Var marked with + vartype = RECORD, + and wholerowN + for a whole-row Var with + vartype equal to the table's declared row type. + Re-use these names when you can (the planner will combine duplicate + requests for identical junk columns). If you need another kind of + junk column besides these, it might be wise to choose a name prefixed + with your extension name, to avoid conflicts against other FDWs. +

+ If the AddForeignUpdateTargets pointer is set to + NULL, no extra target expressions are added. + (This will make it impossible to implement DELETE + operations, though UPDATE may still be feasible if the FDW + relies on an unchanging primary key to identify rows.) +

+List *
+PlanForeignModify(PlannerInfo *root,
+                  ModifyTable *plan,
+                  Index resultRelation,
+                  int subplan_index);
+

+ + Perform any additional planning actions needed for an insert, update, or + delete on a foreign table. This function generates the FDW-private + information that will be attached to the ModifyTable plan + node that performs the update action. This private information must + have the form of a List, and will be delivered to + BeginForeignModify during the execution stage. +

+ root is the planner's global information about the query. + plan is the ModifyTable plan node, which is + complete except for the fdwPrivLists field. + resultRelation identifies the target foreign table by its + range table index. subplan_index identifies which target of + the ModifyTable plan node this is, counting from zero; + use this if you want to index into per-target-relation substructures of the + plan node. +

+ See Section 59.4 for additional information. +

+ If the PlanForeignModify pointer is set to + NULL, no additional plan-time actions are taken, and the + fdw_private list delivered to + BeginForeignModify will be NIL. +

+void
+BeginForeignModify(ModifyTableState *mtstate,
+                   ResultRelInfo *rinfo,
+                   List *fdw_private,
+                   int subplan_index,
+                   int eflags);
+

+ + Begin executing a foreign table modification operation. This routine is + called during executor startup. It should perform any initialization + needed prior to the actual table modifications. Subsequently, + ExecForeignInsert/ExecForeignBatchInsert, + ExecForeignUpdate or + ExecForeignDelete will be called for tuple(s) to be + inserted, updated, or deleted. +

+ mtstate is the overall state of the + ModifyTable plan node being executed; global data about + the plan and execution state is available via this structure. + rinfo is the ResultRelInfo struct describing + the target foreign table. (The ri_FdwState field of + ResultRelInfo is available for the FDW to store any + private state it needs for this operation.) + fdw_private contains the private data generated by + PlanForeignModify, if any. + subplan_index identifies which target of + the ModifyTable plan node this is. + eflags contains flag bits describing the executor's + operating mode for this plan node. +

+ If the BeginForeignModify pointer is set to + NULL, no action is taken during executor startup. +

+TupleTableSlot *
+ExecForeignInsert(EState *estate,
+                  ResultRelInfo *rinfo,
+                  TupleTableSlot *slot,
+                  TupleTableSlot *planSlot);
+

+ + Insert one tuple into the foreign table. + estate is global execution state for the query. + rinfo is the ResultRelInfo struct describing + the target foreign table. + slot contains the tuple to be inserted; it will match the + row-type definition of the foreign table. + planSlot contains the tuple that was generated by the + ModifyTable plan node's subplan; it differs from + slot in possibly containing additional “junk” + columns. (The planSlot is typically of little interest + for INSERT cases, but is provided for completeness.) +

+ The return value is either a slot containing the data that was actually + inserted (this might differ from the data supplied, for example as a + result of trigger actions), or NULL if no row was actually inserted + (again, typically as a result of triggers). The passed-in + slot can be re-used for this purpose. +

+ The data in the returned slot is used only if the INSERT + statement has a RETURNING clause or involves a view + WITH CHECK OPTION; or if the foreign table has + an AFTER ROW trigger. Triggers require all columns, + but the FDW could choose to optimize away returning some or all columns + depending on the contents of the RETURNING clause or + WITH CHECK OPTION constraints. Regardless, some slot + must be returned to indicate success, or the query's reported row count + will be wrong. +

+ If the ExecForeignInsert pointer is set to + NULL, attempts to insert into the foreign table will fail + with an error message. +

+ Note that this function is also called when inserting routed tuples into + a foreign-table partition or executing COPY FROM on + a foreign table, in which case it is called in a different way than it + is in the INSERT case. See the callback functions + described below that allow the FDW to support that. +

+TupleTableSlot **
+ExecForeignBatchInsert(EState *estate,
+                       ResultRelInfo *rinfo,
+                       TupleTableSlot **slots,
+                       TupleTableSlot **planSlots,
+                       int *numSlots);
+

+ + Insert multiple tuples in bulk into the foreign table. + The parameters are the same for ExecForeignInsert + except slots and planSlots contain + multiple tuples and *numSlots specifies the number of + tuples in those arrays. +

+ The return value is an array of slots containing the data that was + actually inserted (this might differ from the data supplied, for + example as a result of trigger actions.) + The passed-in slots can be re-used for this purpose. + The number of successfully inserted tuples is returned in + *numSlots. +

+ The data in the returned slot is used only if the INSERT + statement involves a view + WITH CHECK OPTION; or if the foreign table has + an AFTER ROW trigger. Triggers require all columns, + but the FDW could choose to optimize away returning some or all columns + depending on the contents of the + WITH CHECK OPTION constraints. +

+ If the ExecForeignBatchInsert or + GetForeignModifyBatchSize pointer is set to + NULL, attempts to insert into the foreign table will + use ExecForeignInsert. + This function is not used if the INSERT has the + RETURNING clause. +

+ Note that this function is also called when inserting routed tuples into + a foreign-table partition. See the callback functions + described below that allow the FDW to support that. +

+int
+GetForeignModifyBatchSize(ResultRelInfo *rinfo);
+

+ + Report the maximum number of tuples that a single + ExecForeignBatchInsert call can handle for + the specified foreign table. The executor passes at most + the given number of tuples to ExecForeignBatchInsert. + rinfo is the ResultRelInfo struct describing + the target foreign table. + The FDW is expected to provide a foreign server and/or foreign + table option for the user to set this value, or some hard-coded value. +

+ If the ExecForeignBatchInsert or + GetForeignModifyBatchSize pointer is set to + NULL, attempts to insert into the foreign table will + use ExecForeignInsert. +

+TupleTableSlot *
+ExecForeignUpdate(EState *estate,
+                  ResultRelInfo *rinfo,
+                  TupleTableSlot *slot,
+                  TupleTableSlot *planSlot);
+

+ + Update one tuple in the foreign table. + estate is global execution state for the query. + rinfo is the ResultRelInfo struct describing + the target foreign table. + slot contains the new data for the tuple; it will match the + row-type definition of the foreign table. + planSlot contains the tuple that was generated by the + ModifyTable plan node's subplan. Unlike + slot, this tuple contains only the new values for + columns changed by the query, so do not rely on attribute numbers of the + foreign table to index into planSlot. + Also, planSlot typically contains + additional “junk” columns. In particular, any junk columns + that were requested by AddForeignUpdateTargets will + be available from this slot. +

+ The return value is either a slot containing the row as it was actually + updated (this might differ from the data supplied, for example as a + result of trigger actions), or NULL if no row was actually updated + (again, typically as a result of triggers). The passed-in + slot can be re-used for this purpose. +

+ The data in the returned slot is used only if the UPDATE + statement has a RETURNING clause or involves a view + WITH CHECK OPTION; or if the foreign table has + an AFTER ROW trigger. Triggers require all columns, + but the FDW could choose to optimize away returning some or all columns + depending on the contents of the RETURNING clause or + WITH CHECK OPTION constraints. Regardless, some slot + must be returned to indicate success, or the query's reported row count + will be wrong. +

+ If the ExecForeignUpdate pointer is set to + NULL, attempts to update the foreign table will fail + with an error message. +

+TupleTableSlot *
+ExecForeignDelete(EState *estate,
+                  ResultRelInfo *rinfo,
+                  TupleTableSlot *slot,
+                  TupleTableSlot *planSlot);
+

+ + Delete one tuple from the foreign table. + estate is global execution state for the query. + rinfo is the ResultRelInfo struct describing + the target foreign table. + slot contains nothing useful upon call, but can be used to + hold the returned tuple. + planSlot contains the tuple that was generated by the + ModifyTable plan node's subplan; in particular, it will + carry any junk columns that were requested by + AddForeignUpdateTargets. The junk column(s) must be used + to identify the tuple to be deleted. +

+ The return value is either a slot containing the row that was deleted, + or NULL if no row was deleted (typically as a result of triggers). The + passed-in slot can be used to hold the tuple to be returned. +

+ The data in the returned slot is used only if the DELETE + query has a RETURNING clause or the foreign table has + an AFTER ROW trigger. Triggers require all columns, but the + FDW could choose to optimize away returning some or all columns depending + on the contents of the RETURNING clause. Regardless, some + slot must be returned to indicate success, or the query's reported row + count will be wrong. +

+ If the ExecForeignDelete pointer is set to + NULL, attempts to delete from the foreign table will fail + with an error message. +

+void
+EndForeignModify(EState *estate,
+                 ResultRelInfo *rinfo);
+

+ + End the table update 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. +

+ If the EndForeignModify pointer is set to + NULL, no action is taken during executor shutdown. +

+ Tuples inserted into a partitioned table by INSERT or + COPY FROM are routed to partitions. If an FDW + supports routable foreign-table partitions, it should also provide the + following callback functions. These functions are also called when + COPY FROM is executed on a foreign table. +

+void
+BeginForeignInsert(ModifyTableState *mtstate,
+                   ResultRelInfo *rinfo);
+

+ + Begin executing an insert operation on a foreign table. This routine is + called right before the first tuple is inserted into the foreign table + in both cases when it is the partition chosen for tuple routing and the + target specified in a COPY FROM command. It should + perform any initialization needed prior to the actual insertion. + Subsequently, ExecForeignInsert or + ExecForeignBatchInsert will be called for + tuple(s) to be inserted into the foreign table. +

+ When this is called by a COPY FROM command, the + plan-related global data in mtstate is not provided + and the planSlot parameter of + ExecForeignInsert subsequently called for each + inserted tuple is NULL, whether the foreign table is + the partition chosen for tuple routing or the target specified in the + command. +

+ If the BeginForeignInsert pointer is set to + NULL, no action is taken for the initialization. +

+ Note that if the FDW does not support routable foreign-table partitions + and/or executing COPY FROM on foreign tables, this + function or ExecForeignInsert/ExecForeignBatchInsert + subsequently called must throw error as needed. +

+void
+EndForeignInsert(EState *estate,
+                 ResultRelInfo *rinfo);
+

+ + End the insert operation 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. +

+ If the EndForeignInsert pointer is set to + NULL, no action is taken for the termination. +

+int
+IsForeignRelUpdatable(Relation rel);
+

+ + Report which update operations the specified foreign table supports. + The return value should be a bit mask of rule event numbers indicating + which operations are supported by the foreign table, using the + CmdType enumeration; that is, + (1 << CMD_UPDATE) = 4 for UPDATE, + (1 << CMD_INSERT) = 8 for INSERT, and + (1 << CMD_DELETE) = 16 for DELETE. +

+ If the IsForeignRelUpdatable pointer is set to + NULL, foreign tables are assumed to be insertable, updatable, + or deletable if the FDW provides ExecForeignInsert, + ExecForeignUpdate, or ExecForeignDelete + respectively. This function is only needed if the FDW supports some + tables that are updatable and some that are not. (Even then, it's + permissible to throw an error in the execution routine instead of + checking in this function. However, this function is used to determine + updatability for display in the information_schema views.) +

+ Some inserts, updates, and deletes to foreign tables can be optimized + by implementing an alternative set of interfaces. The ordinary + interfaces for inserts, updates, and deletes fetch rows from the remote + server and then modify those rows one at a time. In some cases, this + row-by-row approach is necessary, but it can be inefficient. If it is + possible for the foreign server to determine which rows should be + modified without actually retrieving them, and if there are no local + structures which would affect the operation (row-level local triggers, + stored generated columns, or WITH CHECK OPTION + constraints from parent views), then it is possible to arrange things + so that the entire operation is performed on the remote server. The + interfaces described below make this possible. +

+bool
+PlanDirectModify(PlannerInfo *root,
+                 ModifyTable *plan,
+                 Index resultRelation,
+                 int subplan_index);
+

+ + Decide whether it is safe to execute a direct modification + on the remote server. If so, return true after performing + planning actions needed for that. Otherwise, return false. + This optional function is called during query planning. + If this function succeeds, BeginDirectModify, + IterateDirectModify and EndDirectModify will + be called at the execution stage, instead. Otherwise, the table + modification will be executed using the table-updating functions + described above. + The parameters are the same as for PlanForeignModify. +

+ To execute the direct modification on the remote server, this function + must rewrite the target subplan with a ForeignScan plan + node that executes the direct modification on the remote server. The + operation and resultRelation fields + of the ForeignScan must be set appropriately. + operation must be set to the CmdType + enumeration corresponding to the statement kind (that is, + CMD_UPDATE for UPDATE, + CMD_INSERT for INSERT, and + CMD_DELETE for DELETE), and the + resultRelation argument must be copied to the + resultRelation field. +

+ See Section 59.4 for additional information. +

+ If the PlanDirectModify pointer is set to + NULL, no attempts to execute a direct modification on the + remote server are taken. +

+void
+BeginDirectModify(ForeignScanState *node,
+                  int eflags);
+

+ + Prepare to execute a direct modification on the remote server. + This is called during executor startup. It should perform any + initialization needed prior to the direct modification (that should be + done upon the first call to IterateDirectModify). + The ForeignScanState node has already been created, but + its fdw_state field is still NULL. Information about + the table to modify is accessible through the + ForeignScanState node (in particular, from the underlying + ForeignScan plan node, which contains any FDW-private + information provided by PlanDirectModify). + eflags contains flag bits describing the executor's + operating mode for this plan node. +

+ If the BeginDirectModify pointer is set to + NULL, no attempts to execute a direct modification on the + remote server are taken. +

+TupleTableSlot *
+IterateDirectModify(ForeignScanState *node);
+

+ + When the INSERT, UPDATE or DELETE + query doesn't have a RETURNING clause, just return NULL + after a direct modification on the remote server. + When the query has the clause, fetch one result containing the data + needed for the RETURNING calculation, returning it in a + tuple table slot (the node's ScanTupleSlot should be + used for this purpose). The data that was actually inserted, updated + or deleted must be stored in + node->resultRelInfo->ri_projectReturning->pi_exprContext->ecxt_scantuple. + Return NULL if no more rows are available. + Note that this is called in a short-lived memory context that will be + reset between invocations. Create a memory context in + BeginDirectModify if you need longer-lived storage, or use + the es_query_cxt of the node's EState. +

+ The rows returned must match the fdw_scan_tlist target + list if one was supplied, otherwise they must match the row type of the + foreign table being updated. If you choose to optimize away fetching + columns that are not needed for the RETURNING calculation, + you should insert nulls in those column positions, or else generate a + fdw_scan_tlist list with those columns omitted. +

+ Whether the query has the clause or not, the query's reported row count + must be incremented by the FDW itself. When the query doesn't have the + clause, the FDW must also increment the row count for the + ForeignScanState node in the EXPLAIN ANALYZE + case. +

+ If the IterateDirectModify pointer is set to + NULL, no attempts to execute a direct modification on the + remote server are taken. +

+void
+EndDirectModify(ForeignScanState *node);
+

+ + Clean up following a direct modification on the remote server. It is + normally not important to release palloc'd memory, but for example open + files and connections to the remote server should be cleaned up. +

+ If the EndDirectModify pointer is set to + NULL, no attempts to execute a direct modification on the + remote server are taken. +

59.2.5. FDW Routines for `TRUNCATE`

+void
+ExecForeignTruncate(List *rels,
+                    DropBehavior behavior,
+                    bool restart_seqs);
+

+ + Truncate foreign tables. This function is called when + TRUNCATE is executed on a foreign table. + rels is a list of Relation + data structures of foreign tables to truncate. +

+ behavior is either DROP_RESTRICT + or DROP_CASCADE indicating that the + RESTRICT or CASCADE option was + requested in the original TRUNCATE command, + respectively. +

+ If restart_seqs is true, + the original TRUNCATE command requested the + RESTART IDENTITY behavior, otherwise the + CONTINUE IDENTITY behavior was requested. +

+ Note that the ONLY options specified + in the original TRUNCATE command are not passed to + ExecForeignTruncate. This behavior is similar to + the callback functions of SELECT, + UPDATE and DELETE on + a foreign table. +

+ ExecForeignTruncate is invoked once per + foreign server for which foreign tables are to be truncated. + This means that all foreign tables included in rels + must belong to the same server. +

+ If the ExecForeignTruncate pointer is set to + NULL, attempts to truncate foreign tables will + fail with an error message. +

59.2.6. FDW Routines for Row Locking

+ If an FDW wishes to support late row locking (as described + in Section 59.5), it must provide the following + callback functions: +

+RowMarkType
+GetForeignRowMarkType(RangeTblEntry *rte,
+                      LockClauseStrength strength);
+

+ + Report which row-marking option to use for a foreign table. + rte is the RangeTblEntry node for the table + and strength describes the lock strength requested by the + relevant FOR UPDATE/SHARE clause, if any. The result must be + a member of the RowMarkType enum type. +

+ This function is called during query planning for each foreign table that + appears in an UPDATE, DELETE, or SELECT + FOR UPDATE/SHARE query and is not the target of UPDATE + or DELETE. +

+ If the GetForeignRowMarkType pointer is set to + NULL, the ROW_MARK_COPY option is always used. + (This implies that RefetchForeignRow will never be called, + so it need not be provided either.) +

+ See Section 59.5 for more information. +

+void
+RefetchForeignRow(EState *estate,
+                  ExecRowMark *erm,
+                  Datum rowid,
+                  TupleTableSlot *slot,
+                  bool *updated);
+

+ + Re-fetch one tuple slot from the foreign table, after locking it if required. + estate is global execution state for the query. + erm is the ExecRowMark struct describing + the target foreign table and the row lock type (if any) to acquire. + rowid identifies the tuple to be fetched. + slot contains nothing useful upon call, but can be used to + hold the returned tuple. updated is an output parameter. +

+ This function should store the tuple into the provided slot, or clear it if + the row lock couldn't be obtained. The row lock type to acquire is + defined by erm->markType, which is the value + previously returned by GetForeignRowMarkType. + (ROW_MARK_REFERENCE means to just re-fetch the tuple + without acquiring any lock, and ROW_MARK_COPY will + never be seen by this routine.) +

+ In addition, *updated should be set to true + if what was fetched was an updated version of the tuple rather than + the same version previously obtained. (If the FDW cannot be sure about + this, always returning true is recommended.) +

+ Note that by default, failure to acquire a row lock should result in + raising an error; returning with an empty slot is only appropriate if + the SKIP LOCKED option is specified + by erm->waitPolicy. +

+ The rowid is the ctid value previously read + for the row to be re-fetched. Although the rowid value is + passed as a Datum, it can currently only be a tid. The + function API is chosen in hopes that it may be possible to allow other + data types for row IDs in future. +

+ If the RefetchForeignRow pointer is set to + NULL, attempts to re-fetch rows will fail + with an error message. +

+ See Section 59.5 for more information. +

+bool
+RecheckForeignScan(ForeignScanState *node,
+                   TupleTableSlot *slot);
+

+ Recheck that a previously-returned tuple still matches the relevant + scan and join qualifiers, and possibly provide a modified version of + the tuple. For foreign data wrappers which do not perform join pushdown, + it will typically be more convenient to set this to NULL and + instead set fdw_recheck_quals appropriately. + When outer joins are pushed down, however, it isn't sufficient to + reapply the checks relevant to all the base tables to the result tuple, + even if all needed attributes are present, because failure to match some + qualifier might result in some attributes going to NULL, rather than in + no tuple being returned. RecheckForeignScan can recheck + qualifiers and return true if they are still satisfied and false + otherwise, but it can also store a replacement tuple into the supplied + slot. +

+ To implement join pushdown, a foreign data wrapper will typically + construct an alternative local join plan which is used only for + rechecks; this will become the outer subplan of the + ForeignScan. When a recheck is required, this subplan + can be executed and the resulting tuple can be stored in the slot. + This plan need not be efficient since no base table will return more + than one row; for example, it may implement all joins as nested loops. + The function GetExistingLocalJoinPath may be used to search + existing paths for a suitable local join path, which can be used as the + alternative local join plan. GetExistingLocalJoinPath + searches for an unparameterized path in the path list of the specified + join relation. (If it does not find such a path, it returns NULL, in + which case a foreign data wrapper may build the local path by itself or + may choose not to create access paths for that join.) +

59.2.7. FDW Routines for `EXPLAIN`

+void
+ExplainForeignScan(ForeignScanState *node,
+                   ExplainState *es);
+

+ + Print additional EXPLAIN output for a foreign table scan. + This function can 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. +

+ If the ExplainForeignScan pointer is set to + NULL, no additional information is printed during + EXPLAIN. +

+void
+ExplainForeignModify(ModifyTableState *mtstate,
+                     ResultRelInfo *rinfo,
+                     List *fdw_private,
+                     int subplan_index,
+                     struct ExplainState *es);
+

+ + Print additional EXPLAIN output for a foreign table update. + This function can 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 ModifyTableState node + can be inspected to provide run-time statistics in the EXPLAIN + ANALYZE case. The first four arguments are the same as for + BeginForeignModify. +

+ If the ExplainForeignModify pointer is set to + NULL, no additional information is printed during + EXPLAIN. +

+void
+ExplainDirectModify(ForeignScanState *node,
+                    ExplainState *es);
+

+ + Print additional EXPLAIN output for a direct modification + on the remote server. + This function can 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. +

+ If the ExplainDirectModify pointer is set to + NULL, no additional information is printed during + EXPLAIN. +

59.2.8. FDW Routines for `ANALYZE`

+bool
+AnalyzeForeignTable(Relation relation,
+                    AcquireSampleRowsFunc *func,
+                    BlockNumber *totalpages);
+

+ + This function is called when ANALYZE 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.) +

59.2.9. FDW Routines for `IMPORT FOREIGN SCHEMA`

+List *
+ImportForeignSchema(ImportForeignSchemaStmt *stmt, Oid serverOid);
+

+ + Obtain a list of foreign table creation commands. This function is + called when executing IMPORT FOREIGN SCHEMA, and is + passed the parse tree for that statement, as well as the OID of the + foreign server to use. It should return a list of C strings, each of + which must contain a CREATE FOREIGN TABLE command. + These strings will be parsed and executed by the core server. +

+ Within the ImportForeignSchemaStmt struct, + remote_schema is the name of the remote schema from + which tables are to be imported. + list_type identifies how to filter table names: + FDW_IMPORT_SCHEMA_ALL means that all tables in the remote + schema should be imported (in this case table_list is + empty), FDW_IMPORT_SCHEMA_LIMIT_TO means to include only + tables listed in table_list, + and FDW_IMPORT_SCHEMA_EXCEPT means to exclude the tables + listed in table_list. + options is a list of options used for the import process. + The meanings of the options are up to the FDW. + For example, an FDW could use an option to define whether the + NOT NULL attributes of columns should be imported. + These options need not have anything to do with those supported by the + FDW as database object options. +

+ The FDW may ignore the local_schema field of + the ImportForeignSchemaStmt, because the core server + will automatically insert that name into the parsed CREATE + FOREIGN TABLE commands. +

+ The FDW does not have to concern itself with implementing the filtering + specified by list_type and table_list, + either, as the core server will automatically skip any returned commands + for tables excluded according to those options. However, it's often + useful to avoid the work of creating commands for excluded tables in the + first place. The function IsImportableForeignTable() may be + useful to test whether a given foreign-table name will pass the filter. +

+ If the FDW does not support importing table definitions, the + ImportForeignSchema pointer can be set to NULL. +

59.2.10. FDW Routines for Parallel Execution

+ A ForeignScan node can, optionally, support parallel + execution. A parallel ForeignScan will be executed + in multiple processes and must return each row exactly once across + all cooperating processes. To do this, processes can coordinate through + fixed-size chunks of dynamic shared memory. This shared memory is not + guaranteed to be mapped at the same address in every process, so it + must not contain pointers. The following functions are all optional, + but most are required if parallel execution is to be supported. +

+bool
+IsForeignScanParallelSafe(PlannerInfo *root, RelOptInfo *rel,
+                          RangeTblEntry *rte);
+

+ Test whether a scan can be performed within a parallel worker. This + function will only be called when the planner believes that a parallel + plan might be possible, and should return true if it is safe for that scan + to run within a parallel worker. This will generally not be the case if + the remote data source has transaction semantics, unless the worker's + connection to the data can somehow be made to share the same transaction + context as the leader. +

+ If this function is not defined, it is assumed that the scan must take + place within the parallel leader. Note that returning true does not mean + that the scan itself can be done in parallel, only that the scan can be + performed within a parallel worker. Therefore, it can be useful to define + this method even when parallel execution is not supported. +

+Size
+EstimateDSMForeignScan(ForeignScanState *node, ParallelContext *pcxt);
+

+ Estimate the amount of dynamic shared memory that will be required + for parallel operation. This may be higher than the amount that will + actually be used, but it must not be lower. The return value is in bytes. + This function is optional, and can be omitted if not needed; but if it + is omitted, the next three functions must be omitted as well, because + no shared memory will be allocated for the FDW's use. +

+void
+InitializeDSMForeignScan(ForeignScanState *node, ParallelContext *pcxt,
+                         void *coordinate);
+

+ Initialize the dynamic shared memory that will be required for parallel + operation. coordinate points to a shared memory area of + size equal to the return value of EstimateDSMForeignScan. + This function is optional, and can be omitted if not needed. +

+void
+ReInitializeDSMForeignScan(ForeignScanState *node, ParallelContext *pcxt,
+                           void *coordinate);
+

+ Re-initialize the dynamic shared memory required for parallel operation + when the foreign-scan plan node is about to be re-scanned. + This function is optional, and can be omitted if not needed. + Recommended practice is that this function reset only shared state, + while the ReScanForeignScan function resets only local + state. Currently, this function will be called + before ReScanForeignScan, but it's best not to rely on + that ordering. +

+void
+InitializeWorkerForeignScan(ForeignScanState *node, shm_toc *toc,
+                            void *coordinate);
+

+ Initialize a parallel worker's local state based on the shared state + set up by the leader during InitializeDSMForeignScan. + This function is optional, and can be omitted if not needed. +

+void
+ShutdownForeignScan(ForeignScanState *node);
+

+ Release resources when it is anticipated the node will not be executed + to completion. This is not called in all cases; sometimes, + EndForeignScan may be called without this function having + been called first. Since the DSM segment used by parallel query is + destroyed just after this callback is invoked, foreign data wrappers that + wish to take some action before the DSM segment goes away should implement + this method. +

59.2.11. FDW Routines for Asynchronous Execution

+ A ForeignScan node can, optionally, support + asynchronous execution as described in + src/backend/executor/README. The following + functions are all optional, but are all required if asynchronous + execution is to be supported. +

+bool
+IsForeignPathAsyncCapable(ForeignPath *path);
+

+ Test whether a given ForeignPath path can scan + the underlying foreign relation asynchronously. + This function will only be called at the end of query planning when the + given path is a direct child of an AppendPath + path and when the planner believes that asynchronous execution improves + performance, and should return true if the given path is able to scan the + foreign relation asynchronously. +

+ If this function is not defined, it is assumed that the given path scans + the foreign relation using IterateForeignScan. + (This implies that the callback functions described below will never be + called, so they need not be provided either.) +

+void
+ForeignAsyncRequest(AsyncRequest *areq);
+

+ Produce one tuple asynchronously from the + ForeignScan node. areq is + the AsyncRequest struct describing the + ForeignScan node and the parent + Append node that requested the tuple from it. + This function should store the tuple into the slot specified by + areq->result, and set + areq->request_complete to true; + or if it needs to wait on an event external to the core server such as + network I/O, and cannot produce any tuple immediately, set the flag to + false, and set + areq->callback_pending to true + for the ForeignScan node to get a callback from + the callback functions described below. If no more tuples are available, + set the slot to NULL or an empty slot, and the + areq->request_complete flag to + true. It's recommended to use + ExecAsyncRequestDone or + ExecAsyncRequestPending to set the output parameters + in the areq. +

+void
+ForeignAsyncConfigureWait(AsyncRequest *areq);
+

+ Configure a file descriptor event for which the + ForeignScan node wishes to wait. + This function will only be called when the + ForeignScan node has the + areq->callback_pending flag set, and should add + the event to the as_eventset of the parent + Append node described by the + areq. See the comments for + ExecAsyncConfigureWait in + src/backend/executor/execAsync.c for additional + information. When the file descriptor event occurs, + ForeignAsyncNotify will be called. +

+void
+ForeignAsyncNotify(AsyncRequest *areq);
+

+ Process a relevant event that has occurred, then produce one tuple + asynchronously from the ForeignScan node. + This function should set the output parameters in the + areq in the same way as + ForeignAsyncRequest. +

59.2.12. FDW Routines for Reparameterization of Paths

+List *
+ReparameterizeForeignPathByChild(PlannerInfo *root, List *fdw_private,
+                                 RelOptInfo *child_rel);
+

+ This function is called while converting a path parameterized by the + top-most parent of the given child relation child_rel to be + parameterized by the child relation. The function is used to reparameterize + any paths or translate any expression nodes saved in the given + fdw_private member of a ForeignPath. The + callback may use reparameterize_path_by_child, + adjust_appendrel_attrs or + adjust_appendrel_attrs_multilevel as required. +

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