# Writing a Foreign Data Wrapper

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. If updating foreign tables is to be supported, the wrapper must handle that, too. 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` subdirectory of the source tree. The sql-createforeigndatawrapper 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.

## Foreign Data Wrapper Functions

The FDW author needs to implement a handler function, and optionally a validator function. Both functions must be written in a compiled language such as C, using the version-1 interface. For details on C language calling conventions and dynamic loading, see xfunc-c.

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 fdw-callbacks.

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. The latter corresponds to the OID of the system catalog the object would be stored in, one of:

- `AttributeRelationId`

- `ForeignDataWrapperRelationId`

- `ForeignServerRelationId`

- `ForeignTableRelationId`

- `UserMappingRelationId`

If no validator function is supplied, options are not checked at object creation time or object alteration time.

## Foreign Data Wrapper Callback Routines

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.

### 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 fdw-planning 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 fdw-planning 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 fdw-planning 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.

### 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.

Note also that the set of join clauses to apply to the join, which is passed as `extra->restrictlist`, varies depending on the combination of inner and outer relations. A ForeignPath path generated for the `joinrel` must contain the set of join clauses it uses, which will be used by the planner to convert the ForeignPath path into a plan, if it is selected by the planner as the best path for the `joinrel`.

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.

Beginning with PostgreSQL 16, fs_relids includes the rangetable indexes of outer joins, if any were involved in this join. The new field fs_base_relids includes only base relation indexes, and thus mimics fs_relids's old semantics.

See fdw-planning for additional information.

### 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 fdw-planning for additional information.

### 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 `ctid`N`` for ctid, `wholerow` for a whole-row Var marked with vartype = `RECORD`, and `wholerow`N`` 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 fdw-planning 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.

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` 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 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.

    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.

`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.)

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 fdw-planning 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 `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.

### FDW Routines for `TRUNCATE`

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

Truncate foreign tables. This function is called when sql-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.

### FDW Routines for Row Locking

If an FDW wishes to support late row locking (as described in fdw-row-locking), 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 fdw-row-locking 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 fdw-row-locking 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.)

### 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`.

### FDW Routines for `ANALYZE`

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

This function is called when sql-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.)

### 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 sql-importforeignschema, 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 sql-createforeigntable 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`.

### 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.

### 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`.

### 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.

## Foreign Data Wrapper Helper Functions

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 *
    GetForeignDataWrapperExtended(Oid fdwid, bits16 flags);

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). flags is a bitwise-or'd bit mask indicating an extra set of options. It can take the value `FDW_MISSING_OK`, in which case a `NULL` result is returned to the caller instead of an error for an undefined object.

    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 *
    GetForeignServerExtended(Oid serverid, bits16 flags);

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). flags is a bitwise-or'd bit mask indicating an extra set of options. It can take the value `FSV_MISSING_OK`, in which case a `NULL` result is returned to the caller instead of an error for an undefined object.

    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 *
    GetForeignColumnOptions(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.

## Foreign Data Wrapper Query Planning

The FDW callback functions `GetForeignRelSize`, `GetForeignPaths`, `GetForeignPlan`, `PlanForeignModify`, `GetForeignJoinPaths`, `GetForeignUpperPaths`, and `PlanDirectModify` 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->reltarget->exprs` 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 `RelOptInfo` 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.

Any clauses removed from the plan node's qual list must instead be added to `fdw_recheck_quals` or rechecked by `RecheckForeignScan` in order to ensure correct behavior at the `READ COMMITTED` isolation level. When a concurrent update occurs for some other table involved in the query, the executor may need to verify that all of the original quals are still satisfied for the tuple, possibly against a different set of parameter values. Using `fdw_recheck_quals` is typically easier than implementing checks inside `RecheckForeignScan`, but this method will be insufficient when outer joins have been pushed down, since the join tuples in that case might have some fields go to NULL without rejecting the tuple entirely.

Another ForeignScan field that can be filled by FDWs is fdw_scan_tlist, which describes the tuples returned by the FDW for this plan node. For simple foreign table scans this can be set to `NIL`, implying that the returned tuples have the row type declared for the foreign table. A non-NIL value must be a target list (list of TargetEntrys) containing Vars and/or expressions representing the returned columns. This might be used, for example, to show that the FDW has omitted some columns that it noticed won't be needed for the query. Also, if the FDW can compute expressions used by the query more cheaply than can be done locally, it could add those expressions to fdw_scan_tlist. Note that join plans (created from paths made by `GetForeignJoinPaths`) must always supply fdw_scan_tlist to describe the set of columns they will return.

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 identify the other relations used in the selected join clause(s) with a suitable value of `param_info`; use `get_baserel_parampathinfo` to compute that value. 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.

If an FDW supports remote joins, `GetForeignJoinPaths` should produce ForeignPaths for potential remote joins in much the same way as `GetForeignPaths` works for base tables. Information about the intended join can be passed forward to `GetForeignPlan` in the same ways described above. However, baserestrictinfo is not relevant for join relations; instead, the relevant join clauses for a particular join are passed to `GetForeignJoinPaths` as a separate parameter (`extra->restrictlist`).

An FDW might additionally support direct execution of some plan actions that are above the level of scans and joins, such as grouping or aggregation. To offer such options, the FDW should generate paths and insert them into the appropriate upper relation. For example, a path representing remote aggregation should be inserted into the `UPPERREL_GROUP_AGG` relation, using `add_path`. This path will be compared on a cost basis with local aggregation performed by reading a simple scan path for the foreign relation (note that such a path must also be supplied, else there will be an error at plan time). If the remote-aggregation path wins, which it usually would, it will be converted into a plan in the usual way, by calling `GetForeignPlan`. The recommended place to generate such paths is in the `GetForeignUpperPaths` callback function, which is called for each upper relation (i.e., each post-scan/join processing step), if all the base relations of the query come from the same FDW.

`PlanForeignModify` and the other callbacks described in fdw-callbacks-update are designed around the assumption that the foreign relation will be scanned in the usual way and then individual row updates will be driven by a local `ModifyTable` plan node. This approach is necessary for the general case where an update requires reading local tables as well as foreign tables. However, if the operation could be executed entirely by the foreign server, the FDW could generate a path representing that and insert it into the `UPPERREL_FINAL` upper relation, where it would compete against the `ModifyTable` approach. This approach could also be used to implement remote `SELECT FOR UPDATE`, rather than using the row locking callbacks described in fdw-callbacks-row-locking. Keep in mind that a path inserted into `UPPERREL_FINAL` is responsible for implementing all behavior of the query.

When planning an `UPDATE` or `DELETE`, `PlanForeignModify` and `PlanDirectModify` can look up the RelOptInfo struct for the foreign table and make use of the `baserel->fdw_private` data previously created by the scan-planning functions. However, in `INSERT` the target table is not scanned so there is no RelOptInfo for it. The List returned by `PlanForeignModify` has the same restrictions as the fdw_private list of a ForeignScan plan node, that is it must contain only structures that `copyObject` knows how to copy.

`INSERT` with an `ON CONFLICT` clause does not support specifying the conflict target, as unique constraints or exclusion constraints on remote tables are not locally known. This in turn implies that `ON CONFLICT DO UPDATE` is not supported, since the specification is mandatory there.

## Row Locking in Foreign Data Wrappers

If an FDW's underlying storage mechanism has a concept of locking individual rows to prevent concurrent updates of those rows, it is usually worthwhile for the FDW to perform row-level locking with as close an approximation as practical to the semantics used in ordinary PostgreSQL tables. There are multiple considerations involved in this.

One key decision to be made is whether to perform early locking or late locking. In early locking, a row is locked when it is first retrieved from the underlying store, while in late locking, the row is locked only when it is known that it needs to be locked. (The difference arises because some rows may be discarded by locally-checked restriction or join conditions.) Early locking is much simpler and avoids extra round trips to a remote store, but it can cause locking of rows that need not have been locked, resulting in reduced concurrency or even unexpected deadlocks. Also, late locking is only possible if the row to be locked can be uniquely re-identified later. Preferably the row identifier should identify a specific version of the row, as PostgreSQL TIDs do.

By default, PostgreSQL ignores locking considerations when interfacing to FDWs, but an FDW can perform early locking without any explicit support from the core code. The API functions described in fdw-callbacks-row-locking, which were added in PostgreSQL 9.5, allow an FDW to use late locking if it wishes.

An additional consideration is that in `READ COMMITTED` isolation mode, PostgreSQL may need to re-check restriction and join conditions against an updated version of some target tuple. Rechecking join conditions requires re-obtaining copies of the non-target rows that were previously joined to the target tuple. When working with standard PostgreSQL tables, this is done by including the TIDs of the non-target tables in the column list projected through the join, and then re-fetching non-target rows when required. This approach keeps the join data set compact, but it requires inexpensive re-fetch capability, as well as a TID that can uniquely identify the row version to be re-fetched. By default, therefore, the approach used with foreign tables is to include a copy of the entire row fetched from a foreign table in the column list projected through the join. This puts no special demands on the FDW but can result in reduced performance of merge and hash joins. An FDW that is capable of meeting the re-fetch requirements can choose to do it the first way.

For an `UPDATE` or `DELETE` on a foreign table, it is recommended that the `ForeignScan` operation on the target table perform early locking on the rows that it fetches, perhaps via the equivalent of `SELECT FOR UPDATE`. An FDW can detect whether a table is an `UPDATE`/`DELETE` target at plan time by comparing its relid to `root->parse->resultRelation`, or at execution time by using `ExecRelationIsTargetRelation()`. An alternative possibility is to perform late locking within the `ExecForeignUpdate` or `ExecForeignDelete` callback, but no special support is provided for this.

For foreign tables that are specified to be locked by a `SELECT FOR UPDATE/SHARE` command, the `ForeignScan` operation can again perform early locking by fetching tuples with the equivalent of `SELECT FOR UPDATE/SHARE`. To perform late locking instead, provide the callback functions defined in fdw-callbacks-row-locking. In `GetForeignRowMarkType`, select rowmark option `ROW_MARK_EXCLUSIVE`, `ROW_MARK_NOKEYEXCLUSIVE`, `ROW_MARK_SHARE`, or `ROW_MARK_KEYSHARE` depending on the requested lock strength. (The core code will act the same regardless of which of these four options you choose.) Elsewhere, you can detect whether a foreign table was specified to be locked by this type of command by using `get_plan_rowmark` at plan time, or `ExecFindRowMark` at execution time; you must check not only whether a non-null rowmark struct is returned, but that its strength field is not `LCS_NONE`.

Lastly, for foreign tables that are used in an `UPDATE`, `DELETE` or `SELECT FOR UPDATE/SHARE` command but are not specified to be row-locked, you can override the default choice to copy entire rows by having `GetForeignRowMarkType` select option `ROW_MARK_REFERENCE` when it sees lock strength `LCS_NONE`. This will cause `RefetchForeignRow` to be called with that value for markType; it should then re-fetch the row without acquiring any new lock. (If you have a `GetForeignRowMarkType` function but don't wish to re-fetch unlocked rows, select option `ROW_MARK_COPY` for `LCS_NONE`.)

See `src/include/nodes/lockoptions.h`, the comments for `RowMarkType` and `PlanRowMark` in `src/include/nodes/plannodes.h`, and the comments for `ExecRowMark` in `src/include/nodes/execnodes.h` for additional information.