Writing a Foreign Data Wrapper {#fdwhandler}
==============================

foreign data wrapper

handler for

All operations on a foreign table are handled through its foreign data
wrapper, which consists of a set of functions that the core server
calls. The foreign data wrapper is responsible for fetching data from
the remote data source and returning it to the PostgreSQL executor. 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.

> **Note**
>
> 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 {#fdw-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 [Foreign Data Wrapper Callback
Routines](#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 (in the
form of the OID of the system catalog the object would be stored in,
either `ForeignDataWrapperRelationId`, `ForeignServerRelationId`,
`UserMappingRelationId`, or `ForeignTableRelationId`). If no validator
function is supplied, options are not checked at object creation time or
object alteration time.

Foreign Data Wrapper Callback Routines {#fdw-callbacks}
======================================

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 {#fdw-callbacks-scan}
----------------------------------------

    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 [Foreign Data Wrapper Query Planning](#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 [Foreign Data Wrapper Query Planning](#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 [Foreign Data Wrapper Query Planning](#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 MDASH 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 {#fdw-callbacks-join-scan}
---------------------------------------

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

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

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

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

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

See [Foreign Data Wrapper Query Planning](#fdw-planning) for additional
information.

FDW Routines for Planning Post-Scan/Join Processing {#fdw-callbacks-upper-planning}
---------------------------------------------------

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 [Foreign Data Wrapper Query Planning](#fdw-planning) for additional
information.

FDW Routines for Updating Foreign Tables {#fdw-callbacks-update}
----------------------------------------

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

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

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

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

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

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

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

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

See [Foreign Data Wrapper Query Planning](#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. 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 [Foreign Data Wrapper Query Planning](#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` {#fdw-callbacks-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 {#fdw-callbacks-row-locking}
----------------------------

If an FDW wishes to support late row locking (as described in [Row
Locking in Foreign Data Wrappers](#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 [Row Locking in Foreign Data Wrappers](#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 [Row Locking in Foreign Data Wrappers](#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` {#fdw-callbacks-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` {#fdw-callbacks-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` {#fdw-callbacks-import}
----------------------------------------

    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 {#fdw-callbacks-parallel}
-----------------------------------

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 {#fdw-callbacks-async}
---------------------------------------

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 {#fdw-callbacks-reparameterize-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 {#fdw-helpers}
=====================================

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 {#fdw-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 Routines
for Updating Foreign Tables](#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 Routines for Row
Locking](#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 {#fdw-row-locking}
====================================

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 Routines
for Row Locking](#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 Routines for Row
Locking](#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.
