Foreign Data Wrapper Callback Routines

55.2.1. 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.

See Section 55.4 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 55.4 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 55.4 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.

55.2.2. 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 , 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 55.4 for additional information.

55.2.3. 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);

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, 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. (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 55.4 for additional information.

55.2.4. 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 (Query *parsetree,
                         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, add TargetEntry items to parsetree->targetList , containing expressions for the extra values to be fetched. Each such entry must be marked resjunk = true , and must have a distinct resname that will identify it at execution time. Avoid using names matching ctid N , wholerow , or wholerow N , as the core system can generate junk columns of these names.

This function is called in the rewriter, not the planner, so the information available is a bit different from that available to the planning routines. parsetree is the parse tree for the UPDATE or DELETE command, while target_rte and target_relation describe the target foreign table.

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 plan->plans or other substructure of the plan node.

See Section 55.4 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 , ExecForeignUpdate or ExecForeignDelete will be called for each tuple 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.

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 ExplainForeignModify and EndForeignModify .

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

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; it differs from slot in possibly containing 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 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 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.

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 triggers which would affect the operation, 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 field of the ForeignScan must be set to the CmdType enumeration appropriately; that is, CMD_UPDATE for UPDATE , CMD_INSERT for INSERT , and CMD_DELETE for DELETE .

See Section 55.4 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.

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 ExplainDirectModify and EndDirectModify .

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 the es_result_relation_info->ri_projectReturning->pi_exprContext->ecxt_scantuple of the node's EState . 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.

55.2.5. FDW Routines For Row Locking

If an FDW wishes to support late row locking (as described in Section 55.5 ), 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 55.5 for more information.

HeapTuple
RefetchForeignRow (EState *estate,
                   ExecRowMark *erm,
                   Datum rowid,
                   bool *updated);

Re-fetch one tuple 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. updated is an output parameter.

This function should return a palloc'ed copy of the fetched tuple, or NULL 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; a NULL return 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 55.5 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.)

55.2.6. 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 .

55.2.7. 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.)

55.2.8. 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 .

55.2.9. FDW Routines for Parallel Execution

A ForeignScan node can, optionally, support parallel execution. A parallel ForeignScan will be executed in multiple processes and should return each row only 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 pointers may not be used. The following callbacks are all optional in general, but 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 callback 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.

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

Initialize the dynamic shared memory that will be required for parallel operation; coordinate points to an amount of allocated space equal to the return value of EstimateDSMForeignScan .

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

Initialize a parallel worker's custom state based on the shared state set up in the leader by InitializeDSMForeignScan . This callback is optional, and needs only be supplied if this custom path supports parallel execution.