<!-- doc/src/sgml/fdwhandler.sgml -->
<chapter id="fdwhandler">
<title>Writing a Foreign Data Wrapper</title>
<indexterm zone="fdwhandler">
<primary>foreign data wrapper</primary>
<secondary>handler for</secondary>
</indexterm>
<para>
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
<productname>PostgreSQL</productname> 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.
</para>
<para>
The foreign data wrappers included in the standard distribution are good
references when trying to write your own. Look into the
<filename>contrib</filename> subdirectory of the source tree.
The <xref linkend="sql-createforeigndatawrapper"/> reference page also has
some useful details.
</para>
<note>
<para>
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.
</para>
</note>
<sect1 id="fdw-functions">
<title>Foreign Data Wrapper Functions</title>
<para>
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 <xref linkend="xfunc-c"/>.
</para>
<para>
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
<productname>PostgreSQL</productname> as taking no arguments and
returning the special pseudo-type <type>fdw_handler</type>. The
callback functions are plain C functions and are not visible or
callable at the SQL level. The callback functions are described in
<xref linkend="fdw-callbacks"/>.
</para>
<para>
The validator function is responsible for validating options given in
<command>CREATE</command> and <command>ALTER</command> 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
<literal>ForeignDataWrapperRelationId</literal>,
<literal>ForeignServerRelationId</literal>,
<literal>UserMappingRelationId</literal>,
or <literal>ForeignTableRelationId</literal>).
If no validator function is supplied, options are not checked at object
creation time or object alteration time.
</para>
</sect1>
<sect1 id="fdw-callbacks">
<title>Foreign Data Wrapper Callback Routines</title>
<para>
The FDW handler function returns a palloc'd <structname>FdwRoutine</structname>
struct containing pointers to the callback functions described below.
The scan-related functions are required, the rest are optional.
</para>
<para>
The <structname>FdwRoutine</structname> struct type is declared in
<filename>src/include/foreign/fdwapi.h</filename>, which see for additional
details.
</para>
<sect2 id="fdw-callbacks-scan">
<title>FDW Routines for Scanning Foreign Tables</title>
<para>
<programlisting>
void
GetForeignRelSize(PlannerInfo *root,
RelOptInfo *baserel,
Oid foreigntableid);
</programlisting>
Obtain relation size estimates for a foreign table. This is called
at the beginning of planning for a query that scans a foreign table.
<literal>root</literal> is the planner's global information about the query;
<literal>baserel</literal> is the planner's information about this table; and
<literal>foreigntableid</literal> is the <structname>pg_class</structname> OID of the
foreign table. (<literal>foreigntableid</literal> could be obtained from the
planner data structures, but it's passed explicitly to save effort.)
</para>
<para>
This function should update <literal>baserel->rows</literal> 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
<literal>baserel->rows</literal> is just a constant default estimate, which
should be replaced if at all possible. The function may also choose to
update <literal>baserel->width</literal> 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 <command>ANALYZE</command>.)
Also, this function may update <literal>baserel->tuples</literal> if
it can compute a better estimate of the foreign table's total row count.
(The initial value is
from <structname>pg_class</structname>.<structfield>reltuples</structfield>
which represents the total row count seen by the
last <command>ANALYZE</command>; it will be <literal>-1</literal> if
no <command>ANALYZE</command> has been done on this foreign table.)
</para>
<para>
See <xref linkend="fdw-planning"/> for additional information.
</para>
<para>
<programlisting>
void
GetForeignPaths(PlannerInfo *root,
RelOptInfo *baserel,
Oid foreigntableid);
</programlisting>
Create possible access paths for a scan on a foreign table.
This is called during query planning.
The parameters are the same as for <function>GetForeignRelSize</function>,
which has already been called.
</para>
<para>
This function must generate at least one access path
(<structname>ForeignPath</structname> node) for a scan on the foreign table and
must call <function>add_path</function> to add each such path to
<literal>baserel->pathlist</literal>. It's recommended to use
<function>create_foreignscan_path</function> to build the
<structname>ForeignPath</structname> nodes. The function can generate multiple
access paths, e.g., a path which has valid <literal>pathkeys</literal> 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.
</para>
<para>
See <xref linkend="fdw-planning"/> for additional information.
</para>
<para>
<programlisting>
ForeignScan *
GetForeignPlan(PlannerInfo *root,
RelOptInfo *baserel,
Oid foreigntableid,
ForeignPath *best_path,
List *tlist,
List *scan_clauses,
Plan *outer_plan);
</programlisting>
Create a <structname>ForeignScan</structname> plan node from the selected foreign
access path. This is called at the end of query planning.
The parameters are as for <function>GetForeignRelSize</function>, plus
the selected <structname>ForeignPath</structname> (previously produced by
<function>GetForeignPaths</function>, <function>GetForeignJoinPaths</function>,
or <function>GetForeignUpperPaths</function>),
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 <structname>ForeignScan</structname>,
which is used for rechecks performed by <function>RecheckForeignScan</function>.
(If the path is for a join rather than a base
relation, <literal>foreigntableid</literal> is <literal>InvalidOid</literal>.)
</para>
<para>
This function must create and return a <structname>ForeignScan</structname> plan
node; it's recommended to use <function>make_foreignscan</function> to build the
<structname>ForeignScan</structname> node.
</para>
<para>
See <xref linkend="fdw-planning"/> for additional information.
</para>
<para>
<programlisting>
void
BeginForeignScan(ForeignScanState *node,
int eflags);
</programlisting>
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 <function>IterateForeignScan</function>).
The <structname>ForeignScanState</structname> node has already been created, but
its <structfield>fdw_state</structfield> field is still NULL. Information about
the table to scan is accessible through the
<structname>ForeignScanState</structname> node (in particular, from the underlying
<structname>ForeignScan</structname> plan node, which contains any FDW-private
information provided by <function>GetForeignPlan</function>).
<literal>eflags</literal> contains flag bits describing the executor's
operating mode for this plan node.
</para>
<para>
Note that when <literal>(eflags & EXEC_FLAG_EXPLAIN_ONLY)</literal> 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 <function>ExplainForeignScan</function> and <function>EndForeignScan</function>.
</para>
<para>
<programlisting>
TupleTableSlot *
IterateForeignScan(ForeignScanState *node);
</programlisting>
Fetch one row from the foreign source, returning it in a tuple table slot
(the node's <structfield>ScanTupleSlot</structfield> 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 <function>BeginForeignScan</function> if you need longer-lived storage, or use
the <structfield>es_query_cxt</structfield> of the node's <structname>EState</structname>.
</para>
<para>
The rows returned must match the <structfield>fdw_scan_tlist</structfield> 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 <structfield>fdw_scan_tlist</structfield> list with
those columns omitted.
</para>
<para>
Note that <productname>PostgreSQL</productname>'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).
</para>
<para>
<programlisting>
void
ReScanForeignScan(ForeignScanState *node);
</programlisting>
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.
</para>
<para>
<programlisting>
void
EndForeignScan(ForeignScanState *node);
</programlisting>
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.
</para>
</sect2>
<sect2 id="fdw-callbacks-join-scan">
<title>FDW Routines for Scanning Foreign Joins</title>
<para>
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:
</para>
<para>
<programlisting>
void
GetForeignJoinPaths(PlannerInfo *root,
RelOptInfo *joinrel,
RelOptInfo *outerrel,
RelOptInfo *innerrel,
JoinType jointype,
JoinPathExtraData *extra);
</programlisting>
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 <function>GetForeignPaths</function>, this function should
generate <structname>ForeignPath</structname> path(s) for the
supplied <literal>joinrel</literal>
(use <function>create_foreign_join_path</function> to build them),
and call <function>add_path</function> to add these
paths to the set of paths considered for the join. But unlike
<function>GetForeignPaths</function>, it is not necessary that this function
succeed in creating at least one path, since paths involving local
joining are always possible.
</para>
<para>
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.
</para>
<para>
If a <structname>ForeignPath</structname> 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 <structfield>scanrelid</structfield> of
the resulting <structname>ForeignScan</structname> plan node should be set to zero,
since there is no single relation that it represents; instead,
the <structfield>fs_relids</structfield> field of the <structname>ForeignScan</structname>
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 <structfield>fdw_scan_tlist</structfield> with an appropriate list
of <structfield>TargetEntry</structfield> nodes, representing the set of columns
it will supply at run time in the tuples it returns.
</para>
<para>
See <xref linkend="fdw-planning"/> for additional information.
</para>
</sect2>
<sect2 id="fdw-callbacks-upper-planning">
<title>FDW Routines for Planning Post-Scan/Join Processing</title>
<para>
If an FDW supports performing remote post-scan/join processing, such as
remote aggregation, it should provide this callback function:
</para>
<para>
<programlisting>
void
GetForeignUpperPaths(PlannerInfo *root,
UpperRelationKind stage,
RelOptInfo *input_rel,
RelOptInfo *output_rel,
void *extra);
</programlisting>
Create possible access paths for <firstterm>upper relation</firstterm> 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 <structname>ForeignPath</structname>
path(s) for any post-scan/join processing that the FDW knows how to
perform remotely
(use <function>create_foreign_upper_path</function> to build them),
and call <function>add_path</function> to add these paths to
the indicated upper relation. As with <function>GetForeignJoinPaths</function>,
it is not necessary that this function succeed in creating any paths,
since paths involving local processing are always possible.
</para>
<para>
The <literal>stage</literal> parameter identifies which post-scan/join step is
currently being considered. <literal>output_rel</literal> is the upper relation
that should receive paths representing computation of this step,
and <literal>input_rel</literal> is the relation representing the input to this
step. The <literal>extra</literal> parameter provides additional details,
currently, it is set only for <literal>UPPERREL_PARTIAL_GROUP_AGG</literal>
or <literal>UPPERREL_GROUP_AGG</literal>, in which case it points to a
<literal>GroupPathExtraData</literal> structure;
or for <literal>UPPERREL_FINAL</literal>, in which case it points to a
<literal>FinalPathExtraData</literal> structure.
(Note that <structname>ForeignPath</structname> paths added
to <literal>output_rel</literal> would typically not have any direct dependency
on paths of the <literal>input_rel</literal>, 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.)
</para>
<para>
See <xref linkend="fdw-planning"/> for additional information.
</para>
</sect2>
<sect2 id="fdw-callbacks-update">
<title>FDW Routines for Updating Foreign Tables</title>
<para>
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:
</para>
<para>
<programlisting>
void
AddForeignUpdateTargets(PlannerInfo *root,
Index rtindex,
RangeTblEntry *target_rte,
Relation target_relation);
</programlisting>
<command>UPDATE</command> and <command>DELETE</command> 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 <quote>junk</quote>, target columns to the list of columns that are to be
retrieved from the foreign table during an <command>UPDATE</command> or
<command>DELETE</command>.
</para>
<para>
To do that, construct a <structname>Var</structname> representing
an extra value you need, and pass it
to <function>add_row_identity_var</function>, 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 <structname>Var</structname> you need, except
that <structname>Var</structname>s that are identical except for
the <structfield>varno</structfield> field can and should share a
column name.
The core system uses the junk column names
<literal>tableoid</literal> for a
table's <structfield>tableoid</structfield> column,
<literal>ctid</literal>
or <literal>ctid<replaceable>N</replaceable></literal>
for <structfield>ctid</structfield>,
<literal>wholerow</literal>
for a whole-row <structname>Var</structname> marked with
<structfield>vartype</structfield> = <type>RECORD</type>,
and <literal>wholerow<replaceable>N</replaceable></literal>
for a whole-row <structname>Var</structname> with
<structfield>vartype</structfield> 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.
</para>
<para>
If the <function>AddForeignUpdateTargets</function> pointer is set to
<literal>NULL</literal>, no extra target expressions are added.
(This will make it impossible to implement <command>DELETE</command>
operations, though <command>UPDATE</command> may still be feasible if the FDW
relies on an unchanging primary key to identify rows.)
</para>
<para>
<programlisting>
List *
PlanForeignModify(PlannerInfo *root,
ModifyTable *plan,
Index resultRelation,
int subplan_index);
</programlisting>
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 <structname>ModifyTable</structname> plan
node that performs the update action. This private information must
have the form of a <literal>List</literal>, and will be delivered to
<function>BeginForeignModify</function> during the execution stage.
</para>
<para>
<literal>root</literal> is the planner's global information about the query.
<literal>plan</literal> is the <structname>ModifyTable</structname> plan node, which is
complete except for the <structfield>fdwPrivLists</structfield> field.
<literal>resultRelation</literal> identifies the target foreign table by its
range table index. <literal>subplan_index</literal> identifies which target of
the <structname>ModifyTable</structname> plan node this is, counting from zero;
use this if you want to index into per-target-relation substructures of the
<literal>plan</literal> node.
</para>
<para>
See <xref linkend="fdw-planning"/> for additional information.
</para>
<para>
If the <function>PlanForeignModify</function> pointer is set to
<literal>NULL</literal>, no additional plan-time actions are taken, and the
<literal>fdw_private</literal> list delivered to
<function>BeginForeignModify</function> will be NIL.
</para>
<para>
<programlisting>
void
BeginForeignModify(ModifyTableState *mtstate,
ResultRelInfo *rinfo,
List *fdw_private,
int subplan_index,
int eflags);
</programlisting>
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,
<function>ExecForeignInsert/ExecForeignBatchInsert</function>,
<function>ExecForeignUpdate</function> or
<function>ExecForeignDelete</function> will be called for tuple(s) to be
inserted, updated, or deleted.
</para>
<para>
<literal>mtstate</literal> is the overall state of the
<structname>ModifyTable</structname> plan node being executed; global data about
the plan and execution state is available via this structure.
<literal>rinfo</literal> is the <structname>ResultRelInfo</structname> struct describing
the target foreign table. (The <structfield>ri_FdwState</structfield> field of
<structname>ResultRelInfo</structname> is available for the FDW to store any
private state it needs for this operation.)
<literal>fdw_private</literal> contains the private data generated by
<function>PlanForeignModify</function>, if any.
<literal>subplan_index</literal> identifies which target of
the <structname>ModifyTable</structname> plan node this is.
<literal>eflags</literal> contains flag bits describing the executor's
operating mode for this plan node.
</para>
<para>
Note that when <literal>(eflags & EXEC_FLAG_EXPLAIN_ONLY)</literal> 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 <function>ExplainForeignModify</function> and <function>EndForeignModify</function>.
</para>
<para>
If the <function>BeginForeignModify</function> pointer is set to
<literal>NULL</literal>, no action is taken during executor startup.
</para>
<para>
<programlisting>
TupleTableSlot *
ExecForeignInsert(EState *estate,
ResultRelInfo *rinfo,
TupleTableSlot *slot,
TupleTableSlot *planSlot);
</programlisting>
Insert one tuple into the foreign table.
<literal>estate</literal> is global execution state for the query.
<literal>rinfo</literal> is the <structname>ResultRelInfo</structname> struct describing
the target foreign table.
<literal>slot</literal> contains the tuple to be inserted; it will match the
row-type definition of the foreign table.
<literal>planSlot</literal> contains the tuple that was generated by the
<structname>ModifyTable</structname> plan node's subplan; it differs from
<literal>slot</literal> in possibly containing additional <quote>junk</quote>
columns. (The <literal>planSlot</literal> is typically of little interest
for <command>INSERT</command> cases, but is provided for completeness.)
</para>
<para>
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
<literal>slot</literal> can be re-used for this purpose.
</para>
<para>
The data in the returned slot is used only if the <command>INSERT</command>
statement has a <literal>RETURNING</literal> clause or involves a view
<literal>WITH CHECK OPTION</literal>; or if the foreign table has
an <literal>AFTER ROW</literal> trigger. Triggers require all columns,
but the FDW could choose to optimize away returning some or all columns
depending on the contents of the <literal>RETURNING</literal> clause or
<literal>WITH CHECK OPTION</literal> constraints. Regardless, some slot
must be returned to indicate success, or the query's reported row count
will be wrong.
</para>
<para>
If the <function>ExecForeignInsert</function> pointer is set to
<literal>NULL</literal>, attempts to insert into the foreign table will fail
with an error message.
</para>
<para>
Note that this function is also called when inserting routed tuples into
a foreign-table partition or executing <command>COPY FROM</command> on
a foreign table, in which case it is called in a different way than it
is in the <command>INSERT</command> case. See the callback functions
described below that allow the FDW to support that.
</para>
<para>
<programlisting>
TupleTableSlot **
ExecForeignBatchInsert(EState *estate,
ResultRelInfo *rinfo,
TupleTableSlot **slots,
TupleTableSlot **planSlots,
int *numSlots);
</programlisting>
Insert multiple tuples in bulk into the foreign table.
The parameters are the same for <function>ExecForeignInsert</function>
except <literal>slots</literal> and <literal>planSlots</literal> contain
multiple tuples and <literal>*numSlots</literal> specifies the number of
tuples in those arrays.
</para>
<para>
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 <literal>slots</literal> can be re-used for this purpose.
The number of successfully inserted tuples is returned in
<literal>*numSlots</literal>.
</para>
<para>
The data in the returned slot is used only if the <command>INSERT</command>
statement involves a view
<literal>WITH CHECK OPTION</literal>; or if the foreign table has
an <literal>AFTER ROW</literal> trigger. Triggers require all columns,
but the FDW could choose to optimize away returning some or all columns
depending on the contents of the
<literal>WITH CHECK OPTION</literal> constraints.
</para>
<para>
If the <function>ExecForeignBatchInsert</function> or
<function>GetForeignModifyBatchSize</function> pointer is set to
<literal>NULL</literal>, attempts to insert into the foreign table will
use <function>ExecForeignInsert</function>.
This function is not used if the <command>INSERT</command> has the
<literal>RETURNING</literal> clause.
</para>
<para>
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.
</para>
<para>
<programlisting>
int
GetForeignModifyBatchSize(ResultRelInfo *rinfo);
</programlisting>
Report the maximum number of tuples that a single
<function>ExecForeignBatchInsert</function> call can handle for
the specified foreign table. The executor passes at most
the given number of tuples to <function>ExecForeignBatchInsert</function>.
<literal>rinfo</literal> is the <structname>ResultRelInfo</structname> 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.
</para>
<para>
If the <function>ExecForeignBatchInsert</function> or
<function>GetForeignModifyBatchSize</function> pointer is set to
<literal>NULL</literal>, attempts to insert into the foreign table will
use <function>ExecForeignInsert</function>.
</para>
<para>
<programlisting>
TupleTableSlot *
ExecForeignUpdate(EState *estate,
ResultRelInfo *rinfo,
TupleTableSlot *slot,
TupleTableSlot *planSlot);
</programlisting>
Update one tuple in the foreign table.
<literal>estate</literal> is global execution state for the query.
<literal>rinfo</literal> is the <structname>ResultRelInfo</structname> struct describing
the target foreign table.
<literal>slot</literal> contains the new data for the tuple; it will match the
row-type definition of the foreign table.
<literal>planSlot</literal> contains the tuple that was generated by the
<structname>ModifyTable</structname> plan node's subplan. Unlike
<literal>slot</literal>, 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 <literal>planSlot</literal>.
Also, <literal>planSlot</literal> typically contains
additional <quote>junk</quote> columns. In particular, any junk columns
that were requested by <function>AddForeignUpdateTargets</function> will
be available from this slot.
</para>
<para>
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
<literal>slot</literal> can be re-used for this purpose.
</para>
<para>
The data in the returned slot is used only if the <command>UPDATE</command>
statement has a <literal>RETURNING</literal> clause or involves a view
<literal>WITH CHECK OPTION</literal>; or if the foreign table has
an <literal>AFTER ROW</literal> trigger. Triggers require all columns,
but the FDW could choose to optimize away returning some or all columns
depending on the contents of the <literal>RETURNING</literal> clause or
<literal>WITH CHECK OPTION</literal> constraints. Regardless, some slot
must be returned to indicate success, or the query's reported row count
will be wrong.
</para>
<para>
If the <function>ExecForeignUpdate</function> pointer is set to
<literal>NULL</literal>, attempts to update the foreign table will fail
with an error message.
</para>
<para>
<programlisting>
TupleTableSlot *
ExecForeignDelete(EState *estate,
ResultRelInfo *rinfo,
TupleTableSlot *slot,
TupleTableSlot *planSlot);
</programlisting>
Delete one tuple from the foreign table.
<literal>estate</literal> is global execution state for the query.
<literal>rinfo</literal> is the <structname>ResultRelInfo</structname> struct describing
the target foreign table.
<literal>slot</literal> contains nothing useful upon call, but can be used to
hold the returned tuple.
<literal>planSlot</literal> contains the tuple that was generated by the
<structname>ModifyTable</structname> plan node's subplan; in particular, it will
carry any junk columns that were requested by
<function>AddForeignUpdateTargets</function>. The junk column(s) must be used
to identify the tuple to be deleted.
</para>
<para>
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 <literal>slot</literal> can be used to hold the tuple to be returned.
</para>
<para>
The data in the returned slot is used only if the <command>DELETE</command>
query has a <literal>RETURNING</literal> clause or the foreign table has
an <literal>AFTER ROW</literal> trigger. Triggers require all columns, but the
FDW could choose to optimize away returning some or all columns depending
on the contents of the <literal>RETURNING</literal> clause. Regardless, some
slot must be returned to indicate success, or the query's reported row
count will be wrong.
</para>
<para>
If the <function>ExecForeignDelete</function> pointer is set to
<literal>NULL</literal>, attempts to delete from the foreign table will fail
with an error message.
</para>
<para>
<programlisting>
void
EndForeignModify(EState *estate,
ResultRelInfo *rinfo);
</programlisting>
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.
</para>
<para>
If the <function>EndForeignModify</function> pointer is set to
<literal>NULL</literal>, no action is taken during executor shutdown.
</para>
<para>
Tuples inserted into a partitioned table by <command>INSERT</command> or
<command>COPY FROM</command> 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
<command>COPY FROM</command> is executed on a foreign table.
</para>
<para>
<programlisting>
void
BeginForeignInsert(ModifyTableState *mtstate,
ResultRelInfo *rinfo);
</programlisting>
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 <command>COPY FROM</command> command. It should
perform any initialization needed prior to the actual insertion.
Subsequently, <function>ExecForeignInsert</function> or
<function>ExecForeignBatchInsert</function> will be called for
tuple(s) to be inserted into the foreign table.
</para>
<para>
<literal>mtstate</literal> is the overall state of the
<structname>ModifyTable</structname> plan node being executed; global data about
the plan and execution state is available via this structure.
<literal>rinfo</literal> is the <structname>ResultRelInfo</structname> struct describing
the target foreign table. (The <structfield>ri_FdwState</structfield> field of
<structname>ResultRelInfo</structname> is available for the FDW to store any
private state it needs for this operation.)
</para>
<para>
When this is called by a <command>COPY FROM</command> command, the
plan-related global data in <literal>mtstate</literal> is not provided
and the <literal>planSlot</literal> parameter of
<function>ExecForeignInsert</function> subsequently called for each
inserted tuple is <literal>NULL</literal>, whether the foreign table is
the partition chosen for tuple routing or the target specified in the
command.
</para>
<para>
If the <function>BeginForeignInsert</function> pointer is set to
<literal>NULL</literal>, no action is taken for the initialization.
</para>
<para>
Note that if the FDW does not support routable foreign-table partitions
and/or executing <command>COPY FROM</command> on foreign tables, this
function or <function>ExecForeignInsert/ExecForeignBatchInsert</function>
subsequently called must throw error as needed.
</para>
<para>
<programlisting>
void
EndForeignInsert(EState *estate,
ResultRelInfo *rinfo);
</programlisting>
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.
</para>
<para>
If the <function>EndForeignInsert</function> pointer is set to
<literal>NULL</literal>, no action is taken for the termination.
</para>
<para>
<programlisting>
int
IsForeignRelUpdatable(Relation rel);
</programlisting>
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
<literal>CmdType</literal> enumeration; that is,
<literal>(1 << CMD_UPDATE) = 4</literal> for <command>UPDATE</command>,
<literal>(1 << CMD_INSERT) = 8</literal> for <command>INSERT</command>, and
<literal>(1 << CMD_DELETE) = 16</literal> for <command>DELETE</command>.
</para>
<para>
If the <function>IsForeignRelUpdatable</function> pointer is set to
<literal>NULL</literal>, foreign tables are assumed to be insertable, updatable,
or deletable if the FDW provides <function>ExecForeignInsert</function>,
<function>ExecForeignUpdate</function>, or <function>ExecForeignDelete</function>
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 <literal>information_schema</literal> views.)
</para>
<para>
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 <literal>WITH CHECK OPTION</literal>
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.
</para>
<para>
<programlisting>
bool
PlanDirectModify(PlannerInfo *root,
ModifyTable *plan,
Index resultRelation,
int subplan_index);
</programlisting>
Decide whether it is safe to execute a direct modification
on the remote server. If so, return <literal>true</literal> after performing
planning actions needed for that. Otherwise, return <literal>false</literal>.
This optional function is called during query planning.
If this function succeeds, <function>BeginDirectModify</function>,
<function>IterateDirectModify</function> and <function>EndDirectModify</function> 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 <function>PlanForeignModify</function>.
</para>
<para>
To execute the direct modification on the remote server, this function
must rewrite the target subplan with a <structname>ForeignScan</structname> plan
node that executes the direct modification on the remote server. The
<structfield>operation</structfield> and <structfield>resultRelation</structfield> fields
of the <structname>ForeignScan</structname> must be set appropriately.
<structfield>operation</structfield> must be set to the <literal>CmdType</literal>
enumeration corresponding to the statement kind (that is,
<literal>CMD_UPDATE</literal> for <command>UPDATE</command>,
<literal>CMD_INSERT</literal> for <command>INSERT</command>, and
<literal>CMD_DELETE</literal> for <command>DELETE</command>), and the
<literal>resultRelation</literal> argument must be copied to the
<structfield>resultRelation</structfield> field.
</para>
<para>
See <xref linkend="fdw-planning"/> for additional information.
</para>
<para>
If the <function>PlanDirectModify</function> pointer is set to
<literal>NULL</literal>, no attempts to execute a direct modification on the
remote server are taken.
</para>
<para>
<programlisting>
void
BeginDirectModify(ForeignScanState *node,
int eflags);
</programlisting>
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 <function>IterateDirectModify</function>).
The <structname>ForeignScanState</structname> node has already been created, but
its <structfield>fdw_state</structfield> field is still NULL. Information about
the table to modify is accessible through the
<structname>ForeignScanState</structname> node (in particular, from the underlying
<structname>ForeignScan</structname> plan node, which contains any FDW-private
information provided by <function>PlanDirectModify</function>).
<literal>eflags</literal> contains flag bits describing the executor's
operating mode for this plan node.
</para>
<para>
Note that when <literal>(eflags & EXEC_FLAG_EXPLAIN_ONLY)</literal> 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 <function>ExplainDirectModify</function> and <function>EndDirectModify</function>.
</para>
<para>
If the <function>BeginDirectModify</function> pointer is set to
<literal>NULL</literal>, no attempts to execute a direct modification on the
remote server are taken.
</para>
<para>
<programlisting>
TupleTableSlot *
IterateDirectModify(ForeignScanState *node);
</programlisting>
When the <command>INSERT</command>, <command>UPDATE</command> or <command>DELETE</command>
query doesn't have a <literal>RETURNING</literal> 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 <literal>RETURNING</literal> calculation, returning it in a
tuple table slot (the node's <structfield>ScanTupleSlot</structfield> should be
used for this purpose). The data that was actually inserted, updated
or deleted must be stored in
<literal>node->resultRelInfo->ri_projectReturning->pi_exprContext->ecxt_scantuple</literal>.
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
<function>BeginDirectModify</function> if you need longer-lived storage, or use
the <structfield>es_query_cxt</structfield> of the node's <structname>EState</structname>.
</para>
<para>
The rows returned must match the <structfield>fdw_scan_tlist</structfield> 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 <literal>RETURNING</literal> calculation,
you should insert nulls in those column positions, or else generate a
<structfield>fdw_scan_tlist</structfield> list with those columns omitted.
</para>
<para>
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
<structname>ForeignScanState</structname> node in the <command>EXPLAIN ANALYZE</command>
case.
</para>
<para>
If the <function>IterateDirectModify</function> pointer is set to
<literal>NULL</literal>, no attempts to execute a direct modification on the
remote server are taken.
</para>
<para>
<programlisting>
void
EndDirectModify(ForeignScanState *node);
</programlisting>
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.
</para>
<para>
If the <function>EndDirectModify</function> pointer is set to
<literal>NULL</literal>, no attempts to execute a direct modification on the
remote server are taken.
</para>
</sect2>
<sect2 id="fdw-callbacks-truncate">
<title>FDW Routines for <command>TRUNCATE</command></title>
<para>
<programlisting>
void
ExecForeignTruncate(List *rels,
DropBehavior behavior,
bool restart_seqs);
</programlisting>
Truncate foreign tables. This function is called when
<xref linkend="sql-truncate"/> is executed on a foreign table.
<literal>rels</literal> is a list of <structname>Relation</structname>
data structures of foreign tables to truncate.
</para>
<para>
<literal>behavior</literal> is either <literal>DROP_RESTRICT</literal>
or <literal>DROP_CASCADE</literal> indicating that the
<literal>RESTRICT</literal> or <literal>CASCADE</literal> option was
requested in the original <command>TRUNCATE</command> command,
respectively.
</para>
<para>
If <literal>restart_seqs</literal> is <literal>true</literal>,
the original <command>TRUNCATE</command> command requested the
<literal>RESTART IDENTITY</literal> behavior, otherwise the
<literal>CONTINUE IDENTITY</literal> behavior was requested.
</para>
<para>
Note that the <literal>ONLY</literal> options specified
in the original <command>TRUNCATE</command> command are not passed to
<function>ExecForeignTruncate</function>. This behavior is similar to
the callback functions of <command>SELECT</command>,
<command>UPDATE</command> and <command>DELETE</command> on
a foreign table.
</para>
<para>
<function>ExecForeignTruncate</function> is invoked once per
foreign server for which foreign tables are to be truncated.
This means that all foreign tables included in <literal>rels</literal>
must belong to the same server.
</para>
<para>
If the <function>ExecForeignTruncate</function> pointer is set to
<literal>NULL</literal>, attempts to truncate foreign tables will
fail with an error message.
</para>
</sect2>
<sect2 id="fdw-callbacks-row-locking">
<title>FDW Routines for Row Locking</title>
<para>
If an FDW wishes to support <firstterm>late row locking</firstterm> (as described
in <xref linkend="fdw-row-locking"/>), it must provide the following
callback functions:
</para>
<para>
<programlisting>
RowMarkType
GetForeignRowMarkType(RangeTblEntry *rte,
LockClauseStrength strength);
</programlisting>
Report which row-marking option to use for a foreign table.
<literal>rte</literal> is the <structname>RangeTblEntry</structname> node for the table
and <literal>strength</literal> describes the lock strength requested by the
relevant <literal>FOR UPDATE/SHARE</literal> clause, if any. The result must be
a member of the <literal>RowMarkType</literal> enum type.
</para>
<para>
This function is called during query planning for each foreign table that
appears in an <command>UPDATE</command>, <command>DELETE</command>, or <command>SELECT
FOR UPDATE/SHARE</command> query and is not the target of <command>UPDATE</command>
or <command>DELETE</command>.
</para>
<para>
If the <function>GetForeignRowMarkType</function> pointer is set to
<literal>NULL</literal>, the <literal>ROW_MARK_COPY</literal> option is always used.
(This implies that <function>RefetchForeignRow</function> will never be called,
so it need not be provided either.)
</para>
<para>
See <xref linkend="fdw-row-locking"/> for more information.
</para>
<para>
<programlisting>
void
RefetchForeignRow(EState *estate,
ExecRowMark *erm,
Datum rowid,
TupleTableSlot *slot,
bool *updated);
</programlisting>
Re-fetch one tuple slot from the foreign table, after locking it if required.
<literal>estate</literal> is global execution state for the query.
<literal>erm</literal> is the <structname>ExecRowMark</structname> struct describing
the target foreign table and the row lock type (if any) to acquire.
<literal>rowid</literal> identifies the tuple to be fetched.
<literal>slot</literal> contains nothing useful upon call, but can be used to
hold the returned tuple. <literal>updated</literal> is an output parameter.
</para>
<para>
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 <literal>erm->markType</literal>, which is the value
previously returned by <function>GetForeignRowMarkType</function>.
(<literal>ROW_MARK_REFERENCE</literal> means to just re-fetch the tuple
without acquiring any lock, and <literal>ROW_MARK_COPY</literal> will
never be seen by this routine.)
</para>
<para>
In addition, <literal>*updated</literal> should be set to <literal>true</literal>
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 <literal>true</literal> is recommended.)
</para>
<para>
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 <literal>SKIP LOCKED</literal> option is specified
by <literal>erm->waitPolicy</literal>.
</para>
<para>
The <literal>rowid</literal> is the <structfield>ctid</structfield> value previously read
for the row to be re-fetched. Although the <literal>rowid</literal> value is
passed as a <type>Datum</type>, it can currently only be a <type>tid</type>. The
function API is chosen in hopes that it may be possible to allow other
data types for row IDs in future.
</para>
<para>
If the <function>RefetchForeignRow</function> pointer is set to
<literal>NULL</literal>, attempts to re-fetch rows will fail
with an error message.
</para>
<para>
See <xref linkend="fdw-row-locking"/> for more information.
</para>
<para>
<programlisting>
bool
RecheckForeignScan(ForeignScanState *node,
TupleTableSlot *slot);
</programlisting>
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 <literal>NULL</literal> and
instead set <structfield>fdw_recheck_quals</structfield> 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. <literal>RecheckForeignScan</literal> 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.
</para>
<para>
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
<literal>ForeignScan</literal>. 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 <literal>GetExistingLocalJoinPath</literal> may be used to search
existing paths for a suitable local join path, which can be used as the
alternative local join plan. <literal>GetExistingLocalJoinPath</literal>
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.)
</para>
</sect2>
<sect2 id="fdw-callbacks-explain">
<title>FDW Routines for <command>EXPLAIN</command></title>
<para>
<programlisting>
void
ExplainForeignScan(ForeignScanState *node,
ExplainState *es);
</programlisting>
Print additional <command>EXPLAIN</command> output for a foreign table scan.
This function can call <function>ExplainPropertyText</function> and
related functions to add fields to the <command>EXPLAIN</command> output.
The flag fields in <literal>es</literal> can be used to determine what to
print, and the state of the <structname>ForeignScanState</structname> node
can be inspected to provide run-time statistics in the <command>EXPLAIN
ANALYZE</command> case.
</para>
<para>
If the <function>ExplainForeignScan</function> pointer is set to
<literal>NULL</literal>, no additional information is printed during
<command>EXPLAIN</command>.
</para>
<para>
<programlisting>
void
ExplainForeignModify(ModifyTableState *mtstate,
ResultRelInfo *rinfo,
List *fdw_private,
int subplan_index,
struct ExplainState *es);
</programlisting>
Print additional <command>EXPLAIN</command> output for a foreign table update.
This function can call <function>ExplainPropertyText</function> and
related functions to add fields to the <command>EXPLAIN</command> output.
The flag fields in <literal>es</literal> can be used to determine what to
print, and the state of the <structname>ModifyTableState</structname> node
can be inspected to provide run-time statistics in the <command>EXPLAIN
ANALYZE</command> case. The first four arguments are the same as for
<function>BeginForeignModify</function>.
</para>
<para>
If the <function>ExplainForeignModify</function> pointer is set to
<literal>NULL</literal>, no additional information is printed during
<command>EXPLAIN</command>.
</para>
<para>
<programlisting>
void
ExplainDirectModify(ForeignScanState *node,
ExplainState *es);
</programlisting>
Print additional <command>EXPLAIN</command> output for a direct modification
on the remote server.
This function can call <function>ExplainPropertyText</function> and
related functions to add fields to the <command>EXPLAIN</command> output.
The flag fields in <literal>es</literal> can be used to determine what to
print, and the state of the <structname>ForeignScanState</structname> node
can be inspected to provide run-time statistics in the <command>EXPLAIN
ANALYZE</command> case.
</para>
<para>
If the <function>ExplainDirectModify</function> pointer is set to
<literal>NULL</literal>, no additional information is printed during
<command>EXPLAIN</command>.
</para>
</sect2>
<sect2 id="fdw-callbacks-analyze">
<title>FDW Routines for <command>ANALYZE</command></title>
<para>
<programlisting>
bool
AnalyzeForeignTable(Relation relation,
AcquireSampleRowsFunc *func,
BlockNumber *totalpages);
</programlisting>
This function is called when <xref linkend="sql-analyze"/> is executed on
a foreign table. If the FDW can collect statistics for this
foreign table, it should return <literal>true</literal>, and provide a pointer
to a function that will collect sample rows from the table in
<parameter>func</parameter>, plus the estimated size of the table in pages in
<parameter>totalpages</parameter>. Otherwise, return <literal>false</literal>.
</para>
<para>
If the FDW does not support collecting statistics for any tables, the
<function>AnalyzeForeignTable</function> pointer can be set to <literal>NULL</literal>.
</para>
<para>
If provided, the sample collection function must have the signature
<programlisting>
int
AcquireSampleRowsFunc(Relation relation,
int elevel,
HeapTuple *rows,
int targrows,
double *totalrows,
double *totaldeadrows);
</programlisting>
A random sample of up to <parameter>targrows</parameter> rows should be collected
from the table and stored into the caller-provided <parameter>rows</parameter>
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 <parameter>totalrows</parameter> and
<parameter>totaldeadrows</parameter>. (Set <parameter>totaldeadrows</parameter> to zero
if the FDW does not have any concept of dead rows.)
</para>
</sect2>
<sect2 id="fdw-callbacks-import">
<title>FDW Routines for <command>IMPORT FOREIGN SCHEMA</command></title>
<para>
<programlisting>
List *
ImportForeignSchema(ImportForeignSchemaStmt *stmt, Oid serverOid);
</programlisting>
Obtain a list of foreign table creation commands. This function is
called when executing <xref linkend="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 <xref linkend="sql-createforeigntable"/> command.
These strings will be parsed and executed by the core server.
</para>
<para>
Within the <structname>ImportForeignSchemaStmt</structname> struct,
<structfield>remote_schema</structfield> is the name of the remote schema from
which tables are to be imported.
<structfield>list_type</structfield> identifies how to filter table names:
<literal>FDW_IMPORT_SCHEMA_ALL</literal> means that all tables in the remote
schema should be imported (in this case <structfield>table_list</structfield> is
empty), <literal>FDW_IMPORT_SCHEMA_LIMIT_TO</literal> means to include only
tables listed in <structfield>table_list</structfield>,
and <literal>FDW_IMPORT_SCHEMA_EXCEPT</literal> means to exclude the tables
listed in <structfield>table_list</structfield>.
<structfield>options</structfield> 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
<literal>NOT NULL</literal> attributes of columns should be imported.
These options need not have anything to do with those supported by the
FDW as database object options.
</para>
<para>
The FDW may ignore the <structfield>local_schema</structfield> field of
the <structname>ImportForeignSchemaStmt</structname>, because the core server
will automatically insert that name into the parsed <command>CREATE
FOREIGN TABLE</command> commands.
</para>
<para>
The FDW does not have to concern itself with implementing the filtering
specified by <structfield>list_type</structfield> and <structfield>table_list</structfield>,
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 <function>IsImportableForeignTable()</function> may be
useful to test whether a given foreign-table name will pass the filter.
</para>
<para>
If the FDW does not support importing table definitions, the
<function>ImportForeignSchema</function> pointer can be set to <literal>NULL</literal>.
</para>
</sect2>
<sect2 id="fdw-callbacks-parallel">
<title>FDW Routines for Parallel Execution</title>
<para>
A <structname>ForeignScan</structname> node can, optionally, support parallel
execution. A parallel <structname>ForeignScan</structname> 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.
</para>
<para>
<programlisting>
bool
IsForeignScanParallelSafe(PlannerInfo *root, RelOptInfo *rel,
RangeTblEntry *rte);
</programlisting>
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.
</para>
<para>
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.
</para>
<para>
<programlisting>
Size
EstimateDSMForeignScan(ForeignScanState *node, ParallelContext *pcxt);
</programlisting>
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.
</para>
<para>
<programlisting>
void
InitializeDSMForeignScan(ForeignScanState *node, ParallelContext *pcxt,
void *coordinate);
</programlisting>
Initialize the dynamic shared memory that will be required for parallel
operation. <literal>coordinate</literal> points to a shared memory area of
size equal to the return value of <function>EstimateDSMForeignScan</function>.
This function is optional, and can be omitted if not needed.
</para>
<para>
<programlisting>
void
ReInitializeDSMForeignScan(ForeignScanState *node, ParallelContext *pcxt,
void *coordinate);
</programlisting>
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 <function>ReScanForeignScan</function> function resets only local
state. Currently, this function will be called
before <function>ReScanForeignScan</function>, but it's best not to rely on
that ordering.
</para>
<para>
<programlisting>
void
InitializeWorkerForeignScan(ForeignScanState *node, shm_toc *toc,
void *coordinate);
</programlisting>
Initialize a parallel worker's local state based on the shared state
set up by the leader during <function>InitializeDSMForeignScan</function>.
This function is optional, and can be omitted if not needed.
</para>
<para>
<programlisting>
void
ShutdownForeignScan(ForeignScanState *node);
</programlisting>
Release resources when it is anticipated the node will not be executed
to completion. This is not called in all cases; sometimes,
<literal>EndForeignScan</literal> 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.
</para>
</sect2>
<sect2 id="fdw-callbacks-async">
<title>FDW Routines for Asynchronous Execution</title>
<para>
A <structname>ForeignScan</structname> node can, optionally, support
asynchronous execution as described in
<filename>src/backend/executor/README</filename>. The following
functions are all optional, but are all required if asynchronous
execution is to be supported.
</para>
<para>
<programlisting>
bool
IsForeignPathAsyncCapable(ForeignPath *path);
</programlisting>
Test whether a given <structname>ForeignPath</structname> 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 <structname>AppendPath</structname>
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.
</para>
<para>
If this function is not defined, it is assumed that the given path scans
the foreign relation using <function>IterateForeignScan</function>.
(This implies that the callback functions described below will never be
called, so they need not be provided either.)
</para>
<para>
<programlisting>
void
ForeignAsyncRequest(AsyncRequest *areq);
</programlisting>
Produce one tuple asynchronously from the
<structname>ForeignScan</structname> node. <literal>areq</literal> is
the <structname>AsyncRequest</structname> struct describing the
<structname>ForeignScan</structname> node and the parent
<structname>Append</structname> node that requested the tuple from it.
This function should store the tuple into the slot specified by
<literal>areq->result</literal>, and set
<literal>areq->request_complete</literal> to <literal>true</literal>;
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
<literal>false</literal>, and set
<literal>areq->callback_pending</literal> to <literal>true</literal>
for the <structname>ForeignScan</structname> 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
<literal>areq->request_complete</literal> flag to
<literal>true</literal>. It's recommended to use
<function>ExecAsyncRequestDone</function> or
<function>ExecAsyncRequestPending</function> to set the output parameters
in the <literal>areq</literal>.
</para>
<para>
<programlisting>
void
ForeignAsyncConfigureWait(AsyncRequest *areq);
</programlisting>
Configure a file descriptor event for which the
<structname>ForeignScan</structname> node wishes to wait.
This function will only be called when the
<structname>ForeignScan</structname> node has the
<literal>areq->callback_pending</literal> flag set, and should add
the event to the <structfield>as_eventset</structfield> of the parent
<structname>Append</structname> node described by the
<literal>areq</literal>. See the comments for
<function>ExecAsyncConfigureWait</function> in
<filename>src/backend/executor/execAsync.c</filename> for additional
information. When the file descriptor event occurs,
<function>ForeignAsyncNotify</function> will be called.
</para>
<para>
<programlisting>
void
ForeignAsyncNotify(AsyncRequest *areq);
</programlisting>
Process a relevant event that has occurred, then produce one tuple
asynchronously from the <structname>ForeignScan</structname> node.
This function should set the output parameters in the
<literal>areq</literal> in the same way as
<function>ForeignAsyncRequest</function>.
</para>
</sect2>
<sect2 id="fdw-callbacks-reparameterize-paths">
<title>FDW Routines for Reparameterization of Paths</title>
<para>
<programlisting>
List *
ReparameterizeForeignPathByChild(PlannerInfo *root, List *fdw_private,
RelOptInfo *child_rel);
</programlisting>
This function is called while converting a path parameterized by the
top-most parent of the given child relation <literal>child_rel</literal> to be
parameterized by the child relation. The function is used to reparameterize
any paths or translate any expression nodes saved in the given
<literal>fdw_private</literal> member of a <structname>ForeignPath</structname>. The
callback may use <literal>reparameterize_path_by_child</literal>,
<literal>adjust_appendrel_attrs</literal> or
<literal>adjust_appendrel_attrs_multilevel</literal> as required.
</para>
</sect2>
</sect1>
<sect1 id="fdw-helpers">
<title>Foreign Data Wrapper Helper Functions</title>
<para>
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
<filename>foreign/foreign.h</filename> in your source file.
That header also defines the struct types that are returned by
these functions.
</para>
<para>
<programlisting>
ForeignDataWrapper *
GetForeignDataWrapperExtended(Oid fdwid, bits16 flags);
</programlisting>
This function returns a <structname>ForeignDataWrapper</structname>
object for the foreign-data wrapper with the given OID. A
<structname>ForeignDataWrapper</structname> object contains properties
of the FDW (see <filename>foreign/foreign.h</filename> for details).
<structfield>flags</structfield> is a bitwise-or'd bit mask indicating
an extra set of options. It can take the value
<literal>FDW_MISSING_OK</literal>, in which case a <literal>NULL</literal>
result is returned to the caller instead of an error for an undefined
object.
</para>
<para>
<programlisting>
ForeignDataWrapper *
GetForeignDataWrapper(Oid fdwid);
</programlisting>
This function returns a <structname>ForeignDataWrapper</structname>
object for the foreign-data wrapper with the given OID. A
<structname>ForeignDataWrapper</structname> object contains properties
of the FDW (see <filename>foreign/foreign.h</filename> for details).
</para>
<para>
<programlisting>
ForeignServer *
GetForeignServerExtended(Oid serverid, bits16 flags);
</programlisting>
This function returns a <structname>ForeignServer</structname> object
for the foreign server with the given OID. A
<structname>ForeignServer</structname> object contains properties
of the server (see <filename>foreign/foreign.h</filename> for details).
<structfield>flags</structfield> is a bitwise-or'd bit mask indicating
an extra set of options. It can take the value
<literal>FSV_MISSING_OK</literal>, in which case a <literal>NULL</literal>
result is returned to the caller instead of an error for an undefined
object.
</para>
<para>
<programlisting>
ForeignServer *
GetForeignServer(Oid serverid);
</programlisting>
This function returns a <structname>ForeignServer</structname> object
for the foreign server with the given OID. A
<structname>ForeignServer</structname> object contains properties
of the server (see <filename>foreign/foreign.h</filename> for details).
</para>
<para>
<programlisting>
UserMapping *
GetUserMapping(Oid userid, Oid serverid);
</programlisting>
This function returns a <structname>UserMapping</structname> 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
<literal>PUBLIC</literal>, or throw error if there is none.) A
<structname>UserMapping</structname> object contains properties of the
user mapping (see <filename>foreign/foreign.h</filename> for details).
</para>
<para>
<programlisting>
ForeignTable *
GetForeignTable(Oid relid);
</programlisting>
This function returns a <structname>ForeignTable</structname> object for
the foreign table with the given OID. A
<structname>ForeignTable</structname> object contains properties of the
foreign table (see <filename>foreign/foreign.h</filename> for details).
</para>
<para>
<programlisting>
List *
GetForeignColumnOptions(Oid relid, AttrNumber attnum);
</programlisting>
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
<structname>DefElem</structname>. NIL is returned if the column has no
options.
</para>
<para>
Some object types have name-based lookup functions in addition to the
OID-based ones:
</para>
<para>
<programlisting>
ForeignDataWrapper *
GetForeignDataWrapperByName(const char *name, bool missing_ok);
</programlisting>
This function returns a <structname>ForeignDataWrapper</structname>
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.
</para>
<para>
<programlisting>
ForeignServer *
GetForeignServerByName(const char *name, bool missing_ok);
</programlisting>
This function returns a <structname>ForeignServer</structname> 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.
</para>
</sect1>
<sect1 id="fdw-planning">
<title>Foreign Data Wrapper Query Planning</title>
<para>
The FDW callback functions <function>GetForeignRelSize</function>,
<function>GetForeignPaths</function>, <function>GetForeignPlan</function>,
<function>PlanForeignModify</function>, <function>GetForeignJoinPaths</function>,
<function>GetForeignUpperPaths</function>, and <function>PlanDirectModify</function>
must fit into the workings of the <productname>PostgreSQL</productname> planner.
Here are some notes about what they must do.
</para>
<para>
The information in <literal>root</literal> and <literal>baserel</literal> can be used
to reduce the amount of information that has to be fetched from the
foreign table (and therefore reduce the cost).
<literal>baserel->baserestrictinfo</literal> is particularly interesting, as
it contains restriction quals (<literal>WHERE</literal> 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.)
<literal>baserel->reltarget->exprs</literal> 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 <structname>ForeignScan</structname> plan node, not
columns that are used in qual evaluation but not output by the query.
</para>
<para>
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.
</para>
<para>
<literal>baserel->fdw_private</literal> is a <type>void</type> 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 <literal>RelOptInfo</literal> node is created.
It is useful for passing information forward from
<function>GetForeignRelSize</function> to <function>GetForeignPaths</function> and/or
<function>GetForeignPaths</function> to <function>GetForeignPlan</function>, thereby
avoiding recalculation.
</para>
<para>
<function>GetForeignPaths</function> can identify the meaning of different
access paths by storing private information in the
<structfield>fdw_private</structfield> field of <structname>ForeignPath</structname> nodes.
<structfield>fdw_private</structfield> is declared as a <type>List</type> 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 <function>nodeToString</function>, for use with debugging support available
in the backend.
</para>
<para>
<function>GetForeignPlan</function> can examine the <structfield>fdw_private</structfield>
field of the selected <structname>ForeignPath</structname> node, and can generate
<structfield>fdw_exprs</structfield> and <structfield>fdw_private</structfield> lists to be
placed in the <structname>ForeignScan</structname> plan node, where they will be
available at execution time. Both of these lists must be
represented in a form that <function>copyObject</function> knows how to copy.
The <structfield>fdw_private</structfield> list has no other restrictions and is
not interpreted by the core backend in any way. The
<structfield>fdw_exprs</structfield> 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.
</para>
<para>
In <function>GetForeignPlan</function>, generally the passed-in target list can
be copied into the plan node as-is. The passed <literal>scan_clauses</literal> list
contains the same clauses as <literal>baserel->baserestrictinfo</literal>,
but may be re-ordered for better execution efficiency. In simple cases
the FDW can just strip <structname>RestrictInfo</structname> nodes from the
<literal>scan_clauses</literal> list (using <function>extract_actual_clauses</function>) 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.
</para>
<para>
As an example, the FDW might identify some restriction clauses of the
form <replaceable>foreign_variable</replaceable> <literal>=</literal>
<replaceable>sub_expression</replaceable>, which it determines can be executed on
the remote server given the locally-evaluated value of the
<replaceable>sub_expression</replaceable>. The actual identification of such a
clause should happen during <function>GetForeignPaths</function>, since it would
affect the cost estimate for the path. The path's
<structfield>fdw_private</structfield> field would probably include a pointer to
the identified clause's <structname>RestrictInfo</structname> node. Then
<function>GetForeignPlan</function> would remove that clause from <literal>scan_clauses</literal>,
but add the <replaceable>sub_expression</replaceable> to <structfield>fdw_exprs</structfield>
to ensure that it gets massaged into executable form. It would probably
also put control information into the plan node's
<structfield>fdw_private</structfield> field to tell the execution functions what
to do at run time. The query transmitted to the remote server would
involve something like <literal>WHERE <replaceable>foreign_variable</replaceable> =
$1</literal>, with the parameter value obtained at run time from
evaluation of the <structfield>fdw_exprs</structfield> expression tree.
</para>
<para>
Any clauses removed from the plan node's qual list must instead be added
to <literal>fdw_recheck_quals</literal> or rechecked by
<literal>RecheckForeignScan</literal> in order to ensure correct behavior
at the <literal>READ COMMITTED</literal> 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
<literal>fdw_recheck_quals</literal> is typically easier than implementing checks
inside <literal>RecheckForeignScan</literal>, 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.
</para>
<para>
Another <structname>ForeignScan</structname> field that can be filled by FDWs
is <structfield>fdw_scan_tlist</structfield>, which describes the tuples returned by
the FDW for this plan node. For simple foreign table scans this can be
set to <literal>NIL</literal>, implying that the returned tuples have the
row type declared for the foreign table. A non-<symbol>NIL</symbol> value must be a
target list (list of <structname>TargetEntry</structname>s) 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 <structfield>fdw_scan_tlist</structfield>. Note that join
plans (created from paths made by <function>GetForeignJoinPaths</function>) must
always supply <structfield>fdw_scan_tlist</structfield> to describe the set of
columns they will return.
</para>
<para>
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
<replaceable>foreign_variable</replaceable> <literal>=</literal>
<replaceable>local_variable</replaceable>. Such clauses will not be found in
<literal>baserel->baserestrictinfo</literal> but must be sought in the
relation's join lists. A path using such a clause is called a
<quote>parameterized path</quote>. It must identify the other relations
used in the selected join clause(s) with a suitable value of
<literal>param_info</literal>; use <function>get_baserel_parampathinfo</function>
to compute that value. In <function>GetForeignPlan</function>, the
<replaceable>local_variable</replaceable> portion of the join clause would be added
to <structfield>fdw_exprs</structfield>, and then at run time the case works the
same as for an ordinary restriction clause.
</para>
<para>
If an FDW supports remote joins, <function>GetForeignJoinPaths</function> should
produce <structname>ForeignPath</structname>s for potential remote joins in much
the same way as <function>GetForeignPaths</function> works for base tables.
Information about the intended join can be passed forward
to <function>GetForeignPlan</function> in the same ways described above.
However, <structfield>baserestrictinfo</structfield> is not relevant for join
relations; instead, the relevant join clauses for a particular join are
passed to <function>GetForeignJoinPaths</function> as a separate parameter
(<literal>extra->restrictlist</literal>).
</para>
<para>
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 <firstterm>upper relation</firstterm>. For
example, a path representing remote aggregation should be inserted into
the <literal>UPPERREL_GROUP_AGG</literal> relation, using <function>add_path</function>.
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 <function>GetForeignPlan</function>. The recommended place to generate
such paths is in the <function>GetForeignUpperPaths</function>
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.
</para>
<para>
<function>PlanForeignModify</function> and the other callbacks described in
<xref linkend="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 <literal>ModifyTable</literal>
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 <literal>UPPERREL_FINAL</literal> upper relation, where it would
compete against the <literal>ModifyTable</literal> approach. This approach
could also be used to implement remote <literal>SELECT FOR UPDATE</literal>,
rather than using the row locking callbacks described in
<xref linkend="fdw-callbacks-row-locking"/>. Keep in mind that a path
inserted into <literal>UPPERREL_FINAL</literal> is responsible for
implementing <emphasis>all</emphasis> behavior of the query.
</para>
<para>
When planning an <command>UPDATE</command> or <command>DELETE</command>,
<function>PlanForeignModify</function> and <function>PlanDirectModify</function>
can look up the <structname>RelOptInfo</structname>
struct for the foreign table and make use of the
<literal>baserel->fdw_private</literal> data previously created by the
scan-planning functions. However, in <command>INSERT</command> the target
table is not scanned so there is no <structname>RelOptInfo</structname> for it.
The <structname>List</structname> returned by <function>PlanForeignModify</function> has
the same restrictions as the <structfield>fdw_private</structfield> list of a
<structname>ForeignScan</structname> plan node, that is it must contain only
structures that <function>copyObject</function> knows how to copy.
</para>
<para>
<command>INSERT</command> with an <literal>ON CONFLICT</literal> 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 <literal>ON CONFLICT DO UPDATE</literal> is not supported,
since the specification is mandatory there.
</para>
</sect1>
<sect1 id="fdw-row-locking">
<title>Row Locking in Foreign Data Wrappers</title>
<para>
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 <productname>PostgreSQL</productname> tables. There are multiple
considerations involved in this.
</para>
<para>
One key decision to be made is whether to perform <firstterm>early
locking</firstterm> or <firstterm>late locking</firstterm>. 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 <productname>PostgreSQL</productname> TIDs do.
</para>
<para>
By default, <productname>PostgreSQL</productname> 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 <xref linkend="fdw-callbacks-row-locking"/>, which were added
in <productname>PostgreSQL</productname> 9.5, allow an FDW to use late locking if
it wishes.
</para>
<para>
An additional consideration is that in <literal>READ COMMITTED</literal>
isolation mode, <productname>PostgreSQL</productname> 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 <productname>PostgreSQL</productname> 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.
</para>
<para>
For an <command>UPDATE</command> or <command>DELETE</command> on a foreign table, it
is recommended that the <literal>ForeignScan</literal> operation on the target
table perform early locking on the rows that it fetches, perhaps via the
equivalent of <command>SELECT FOR UPDATE</command>. An FDW can detect whether
a table is an <command>UPDATE</command>/<command>DELETE</command> target at plan time
by comparing its relid to <literal>root->parse->resultRelation</literal>,
or at execution time by using <function>ExecRelationIsTargetRelation()</function>.
An alternative possibility is to perform late locking within the
<function>ExecForeignUpdate</function> or <function>ExecForeignDelete</function>
callback, but no special support is provided for this.
</para>
<para>
For foreign tables that are specified to be locked by a <command>SELECT
FOR UPDATE/SHARE</command> command, the <literal>ForeignScan</literal> operation can
again perform early locking by fetching tuples with the equivalent
of <command>SELECT FOR UPDATE/SHARE</command>. To perform late locking
instead, provide the callback functions defined
in <xref linkend="fdw-callbacks-row-locking"/>.
In <function>GetForeignRowMarkType</function>, select rowmark option
<literal>ROW_MARK_EXCLUSIVE</literal>, <literal>ROW_MARK_NOKEYEXCLUSIVE</literal>,
<literal>ROW_MARK_SHARE</literal>, or <literal>ROW_MARK_KEYSHARE</literal> 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 <function>get_plan_rowmark</function> at
plan time, or <function>ExecFindRowMark</function> at execution time; you must
check not only whether a non-null rowmark struct is returned, but that
its <structfield>strength</structfield> field is not <literal>LCS_NONE</literal>.
</para>
<para>
Lastly, for foreign tables that are used in an <command>UPDATE</command>,
<command>DELETE</command> or <command>SELECT FOR UPDATE/SHARE</command> command but
are not specified to be row-locked, you can override the default choice
to copy entire rows by having <function>GetForeignRowMarkType</function> select
option <literal>ROW_MARK_REFERENCE</literal> when it sees lock strength
<literal>LCS_NONE</literal>. This will cause <function>RefetchForeignRow</function> to
be called with that value for <structfield>markType</structfield>; it should then
re-fetch the row without acquiring any new lock. (If you have
a <function>GetForeignRowMarkType</function> function but don't wish to re-fetch
unlocked rows, select option <literal>ROW_MARK_COPY</literal>
for <literal>LCS_NONE</literal>.)
</para>
<para>
See <filename>src/include/nodes/lockoptions.h</filename>, the comments
for <type>RowMarkType</type> and <type>PlanRowMark</type>
in <filename>src/include/nodes/plannodes.h</filename>, and the comments for
<type>ExecRowMark</type> in <filename>src/include/nodes/execnodes.h</filename> for
additional information.
</para>
</sect1>
</chapter>