<!-- 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</> 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>ForeignServerRelationId</>,
     <literal>UserMappingRelationId</>,
     or <literal>ForeignTableRelationId</>).
     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</>
     struct containing pointers to the callback functions described below.
     The scan-related functions are required, the rest are optional.
    </para>

    <para>
     The <structname>FdwRoutine</> struct type is declared in
     <filename>src/include/foreign/fdwapi.h</>, 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</> is the planner's global information about the query;
     <literal>baserel</> is the planner's information about this table; and
     <literal>foreigntableid</> is the <structname>pg_class</> OID of the
     foreign table.  (<literal>foreigntableid</> could be obtained from the
     planner data structures, but it's passed explicitly to save effort.)
    </para>

    <para>
     This function should update <literal>baserel-&gt;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
     <literal>baserel-&gt;rows</> is just a constant default estimate, which
     should be replaced if at all possible.  The function may also choose to
     update <literal>baserel-&gt;width</> if it can compute a better estimate
     of the average result row width.
    </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</>,
     which has already been called.
    </para>

    <para>
     This function must generate at least one access path
     (<structname>ForeignPath</> node) for a scan on the foreign table and
     must call <function>add_path</> to add each such path to
     <literal>baserel-&gt;pathlist</>.  It's recommended to use
     <function>create_foreignscan_path</> to build the
     <structname>ForeignPath</> nodes.  The function can generate multiple
     access paths, e.g., a path which has valid <literal>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.
    </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</> plan node from the selected foreign
     access path.  This is called at the end of query planning.
     The parameters are as for <function>GetForeignRelSize</>, plus
     the selected <structname>ForeignPath</> (previously produced by
     <function>GetForeignPaths</> or <function>GetForeignJoinPaths</>),
     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</>,
     which is used for rechecks performed by <function>RecheckForeignScan</>.
     (If the path is for a join rather than a base
     relation, <literal>foreigntableid</> is <literal>InvalidOid</>.)
    </para>

    <para>
     This function must create and return a <structname>ForeignScan</> plan
     node; it's recommended to use <function>make_foreignscan</> to build the
     <structname>ForeignScan</> 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</>).
     The <structname>ForeignScanState</> node has already been created, but
     its <structfield>fdw_state</> field is still NULL.  Information about
     the table to scan is accessible through the
     <structname>ForeignScanState</> node (in particular, from the underlying
     <structname>ForeignScan</> plan node, which contains any FDW-private
     information provided by <function>GetForeignPlan</>).
     <literal>eflags</> contains flag bits describing the executor's
     operating mode for this plan node.
    </para>

    <para>
     Note that when <literal>(eflags &amp; 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 <function>ExplainForeignScan</> and <function>EndForeignScan</>.
    </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</> 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</> if you need longer-lived storage, or use
     the <structfield>es_query_cxt</> of the node's <structname>EState</>.
    </para>

    <para>
     The rows returned must match the <structfield>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 <structfield>fdw_scan_tlist</> 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 &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).
    </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</>, this function should
     generate <structname>ForeignPath</> path(s) for the
     supplied <literal>joinrel</>, and call <function>add_path</> to add these
     paths to the set of paths considered for the join.  But unlike
     <function>GetForeignPaths</>, 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</> 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</> of
     the resulting <structname>ForeignScan</> plan node should be set to zero,
     since there is no single relation that it represents; instead,
     the <structfield>fs_relids</> field of the <structname>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 <structfield>fdw_scan_tlist</> with an appropriate list
     of <structfield>TargetEntry</> 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-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 (Query *parsetree,
                         RangeTblEntry *target_rte,
                         Relation target_relation);
</programlisting>

     <command>UPDATE</> and <command>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 <quote>junk</>, target columns to the list of columns that are to be
     retrieved from the foreign table during an <command>UPDATE</> or
     <command>DELETE</>.
    </para>

    <para>
     To do that, add <structname>TargetEntry</> items to
     <literal>parsetree-&gt;targetList</>, containing expressions for the
     extra values to be fetched.  Each such entry must be marked
     <structfield>resjunk</> = <literal>true</>, and must have a distinct
     <structfield>resname</> that will identify it at execution time.
     Avoid using names matching <literal>ctid<replaceable>N</></literal>,
     <literal>wholerow</literal>, or
     <literal>wholerow<replaceable>N</></literal>, as the core system can
     generate junk columns of these names.
    </para>

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

    <para>
     If the <function>AddForeignUpdateTargets</> pointer is set to
     <literal>NULL</>, no extra target expressions are added.
     (This will make it impossible to implement <command>DELETE</>
     operations, though <command>UPDATE</> 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</> plan
     node that performs the update action.  This private information must
     have the form of a <literal>List</>, and will be delivered to
     <function>BeginForeignModify</> during the execution stage.
    </para>

    <para>
     <literal>root</> is the planner's global information about the query.
     <literal>plan</> is the <structname>ModifyTable</> plan node, which is
     complete except for the <structfield>fdwPrivLists</> field.
     <literal>resultRelation</> identifies the target foreign table by its
     range table index.  <literal>subplan_index</> identifies which target of
     the <structname>ModifyTable</> plan node this is, counting from zero;
     use this if you want to index into <literal>plan-&gt;plans</> or other
     substructure of the <literal>plan</> node.
    </para>

    <para>
     See <xref linkend="fdw-planning"> for additional information.
    </para>

    <para>
     If the <function>PlanForeignModify</> pointer is set to
     <literal>NULL</>, no additional plan-time actions are taken, and the
     <literal>fdw_private</> list delivered to
     <function>BeginForeignModify</> 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</>, <function>ExecForeignUpdate</> or
     <function>ExecForeignDelete</> will be called for each tuple to be
     inserted, updated, or deleted.
    </para>

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

    <para>
     Note that when <literal>(eflags &amp; 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 <function>ExplainForeignModify</> and <function>EndForeignModify</>.
    </para>

    <para>
     If the <function>BeginForeignModify</> pointer is set to
     <literal>NULL</>, 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</> is global execution state for the query.
     <literal>rinfo</> is the <structname>ResultRelInfo</> struct describing
     the target foreign table.
     <literal>slot</> contains the tuple to be inserted; it will match the
     row-type definition of the foreign table.
     <literal>planSlot</> contains the tuple that was generated by the
     <structname>ModifyTable</> plan node's subplan; it differs from
     <literal>slot</> in possibly containing additional <quote>junk</>
     columns.  (The <literal>planSlot</> is typically of little interest
     for <command>INSERT</> 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</> can be re-used for this purpose.
    </para>

    <para>
     The data in the returned slot is used only if the <command>INSERT</>
     query has a <literal>RETURNING</> clause or the foreign table has
     an <literal>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 <literal>RETURNING</> 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>ExecForeignInsert</> pointer is set to
     <literal>NULL</>, attempts to insert into the foreign table will fail
     with an error message.
    </para>

    <para>
<programlisting>
TupleTableSlot *
ExecForeignUpdate (EState *estate,
                   ResultRelInfo *rinfo,
                   TupleTableSlot *slot,
                   TupleTableSlot *planSlot);
</programlisting>

     Update one tuple in the foreign table.
     <literal>estate</> is global execution state for the query.
     <literal>rinfo</> is the <structname>ResultRelInfo</> struct describing
     the target foreign table.
     <literal>slot</> contains the new data for the tuple; it will match the
     row-type definition of the foreign table.
     <literal>planSlot</> contains the tuple that was generated by the
     <structname>ModifyTable</> plan node's subplan; it differs from
     <literal>slot</> in possibly containing additional <quote>junk</>
     columns.  In particular, any junk columns that were requested by
     <function>AddForeignUpdateTargets</> 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</> can be re-used for this purpose.
    </para>

    <para>
     The data in the returned slot is used only if the <command>UPDATE</>
     query has a <literal>RETURNING</> clause or the foreign table has
     an <literal>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 <literal>RETURNING</> 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>ExecForeignUpdate</> pointer is set to
     <literal>NULL</>, 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</> is global execution state for the query.
     <literal>rinfo</> is the <structname>ResultRelInfo</> struct describing
     the target foreign table.
     <literal>slot</> contains nothing useful upon call, but can be used to
     hold the returned tuple.
     <literal>planSlot</> contains the tuple that was generated by the
     <structname>ModifyTable</> plan node's subplan; in particular, it will
     carry any junk columns that were requested by
     <function>AddForeignUpdateTargets</>.  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</> 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</>
     query has a <literal>RETURNING</> clause or the foreign table has
     an <literal>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 <literal>RETURNING</> 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</> pointer is set to
     <literal>NULL</>, 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</> pointer is set to
     <literal>NULL</>, no action is taken during executor shutdown.
    </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</> enumeration; that is,
     <literal>(1 << CMD_UPDATE) = 4</> for <command>UPDATE</>,
     <literal>(1 << CMD_INSERT) = 8</> for <command>INSERT</>, and
     <literal>(1 << CMD_DELETE) = 16</> for <command>DELETE</>.
    </para>

    <para>
     If the <function>IsForeignRelUpdatable</> pointer is set to
     <literal>NULL</>, foreign tables are assumed to be insertable, updatable,
     or deletable if the FDW provides <function>ExecForeignInsert</>,
     <function>ExecForeignUpdate</>, or <function>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 <literal>information_schema</> views.)
    </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</> (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</> is the <structname>RangeTblEntry</> node for the table
     and <literal>strength</> describes the lock strength requested by the
     relevant <literal>FOR UPDATE/SHARE</> clause, if any.  The result must be
     a member of the <literal>RowMarkType</> enum type.
    </para>

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

    <para>
     If the <function>GetForeignRowMarkType</> pointer is set to
     <literal>NULL</>, the <literal>ROW_MARK_COPY</> option is always used.
     (This implies that <function>RefetchForeignRow</> 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>
HeapTuple
RefetchForeignRow (EState *estate,
                   ExecRowMark *erm,
                   Datum rowid,
                   bool *updated);
</programlisting>

     Re-fetch one tuple from the foreign table, after locking it if required.
     <literal>estate</> is global execution state for the query.
     <literal>erm</> is the <structname>ExecRowMark</> struct describing
     the target foreign table and the row lock type (if any) to acquire.
     <literal>rowid</> identifies the tuple to be fetched.
     <literal>updated</> is an output parameter.
    </para>

    <para>
     This function should return a palloc'ed copy of the fetched tuple,
     or <literal>NULL</> if the row lock couldn't be obtained.  The row lock
     type to acquire is defined by <literal>erm-&gt;markType</>, which is the
     value previously returned by <function>GetForeignRowMarkType</>.
     (<literal>ROW_MARK_REFERENCE</> means to just re-fetch the tuple without
     acquiring any lock, and <literal>ROW_MARK_COPY</> will never be seen by
     this routine.)
    </para>

    <para>
     In addition, <literal>*updated</> should be set to <literal>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 <literal>true</> is recommended.)
    </para>

    <para>
     Note that by default, failure to acquire a row lock should result in
     raising an error; a <literal>NULL</> return is only appropriate if
     the <literal>SKIP LOCKED</> option is specified
     by <literal>erm-&gt;waitPolicy</>.
    </para>

    <para>
     The <literal>rowid</> is the <structfield>ctid</> value previously read
     for the row to be re-fetched.  Although the <literal>rowid</> value is
     passed as a <type>Datum</>, it can currently only be a <type>tid</>.  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</> pointer is set to
     <literal>NULL</>, 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</> 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</> 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</>.  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</> 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</>
     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</></title>

    <para>
<programlisting>
void
ExplainForeignScan (ForeignScanState *node,
                    ExplainState *es);
</programlisting>

     Print additional <command>EXPLAIN</> output for a foreign table scan.
     This function can call <function>ExplainPropertyText</> and
     related functions to add fields to the <command>EXPLAIN</> output.
     The flag fields in <literal>es</> can be used to determine what to
     print, and the state of the <structname>ForeignScanState</> node
     can be inspected to provide run-time statistics in the <command>EXPLAIN
     ANALYZE</> case.
    </para>

    <para>
     If the <function>ExplainForeignScan</> pointer is set to
     <literal>NULL</>, no additional information is printed during
     <command>EXPLAIN</>.
    </para>

    <para>
<programlisting>
void
ExplainForeignModify (ModifyTableState *mtstate,
                      ResultRelInfo *rinfo,
                      List *fdw_private,
                      int subplan_index,
                      struct ExplainState *es);
</programlisting>

     Print additional <command>EXPLAIN</> output for a foreign table update.
     This function can call <function>ExplainPropertyText</> and
     related functions to add fields to the <command>EXPLAIN</> output.
     The flag fields in <literal>es</> can be used to determine what to
     print, and the state of the <structname>ModifyTableState</> node
     can be inspected to provide run-time statistics in the <command>EXPLAIN
     ANALYZE</> case.  The first four arguments are the same as for
     <function>BeginForeignModify</>.
    </para>

    <para>
     If the <function>ExplainForeignModify</> pointer is set to
     <literal>NULL</>, no additional information is printed during
     <command>EXPLAIN</>.
    </para>

   </sect2>

   <sect2 id="fdw-callbacks-analyze">
    <title>FDW Routines for <command>ANALYZE</></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</>, and provide a pointer
     to a function that will collect sample rows from the table in
     <parameter>func</>, plus the estimated size of the table in pages in
     <parameter>totalpages</>.  Otherwise, return <literal>false</>.
    </para>

    <para>
     If the FDW does not support collecting statistics for any tables, the
     <function>AnalyzeForeignTable</> pointer can be set to <literal>NULL</>.
    </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</> rows should be collected
     from the table and stored into the caller-provided <parameter>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 <parameter>totalrows</> and
     <parameter>totaldeadrows</>.  (Set <parameter>totaldeadrows</> 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</></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</> struct,
     <structfield>remote_schema</> is the name of the remote schema from
     which tables are to be imported.
     <structfield>list_type</> identifies how to filter table names:
     <literal>FDW_IMPORT_SCHEMA_ALL</> means that all tables in the remote
     schema should be imported (in this case <structfield>table_list</> is
     empty), <literal>FDW_IMPORT_SCHEMA_LIMIT_TO</> means to include only
     tables listed in <structfield>table_list</>,
     and <literal>FDW_IMPORT_SCHEMA_EXCEPT</> means to exclude the tables
     listed in <structfield>table_list</>.
     <structfield>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
     <literal>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.
    </para>

    <para>
     The FDW may ignore the <structfield>local_schema</> field of
     the <structname>ImportForeignSchemaStmt</>, because the core server
     will automatically insert that name into the parsed <command>CREATE
     FOREIGN TABLE</> commands.
    </para>

    <para>
     The FDW does not have to concern itself with implementing the filtering
     specified by <structfield>list_type</> and <structfield>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 <function>IsImportableForeignTable()</> 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</> pointer can be set to <literal>NULL</>.
    </para>

   </sect2>

   <sect2 id="fdw-callbacks-parallel">
    <title>FDW Routines for Parallel Execution</title>
    <para>
     A <structname>ForeignScan</> node can, optionally, support parallel
     execution.  A parallel <structname>ForeignScan</> will be executed
     in multiple processes and should return each row only once across
     all cooperating processes.  To do this, processes can coordinate through
     fixed size chunks of dynamic shared memory.  This shared memory is not
     guaranteed to be mapped at the same address in every process, so pointers
     may not be used.  The following callbacks are all optional in general,
     but required if parallel execution is to be supported.
    </para>

    <para>
<programlisting>
Size
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 callback is not defined, it is assumed that the scan must take
    place within the parallel leader.  Note that returning true does not mean
    that the scan itself can be done in parallel, only that the scan can be
    performed within a parallel worker.  Therefore, it can be useful to define
    this method even when parallel execution is not supported.
    </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.
    </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</> points to an amount of allocated space
    equal to the return value of <function>EstimateDSMForeignScan</>.
   </para>

   <para>
<programlisting>
void
InitializeWorkerForeignScan(ForeignScanState *node, shm_toc *toc,
                            void *coordinate);
</programlisting>
    Initialize a parallel worker's custom state based on the shared state
    set up in the leader by <literal>InitializeDSMForeignScan</>.
    This callback is optional, and needs only be supplied if this
    custom path supports parallel execution.
   </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 *
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 *
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</>, 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>
UserMapping *
GetUserMappingById(Oid umid);
</programlisting>

     This function returns the <structname>UserMapping</structname> object for
     the given user mapping OID.  The OID of a user mapping for a foreign scan
     is available in the <structname>RelOptInfo</structname>.
     If there is no mapping for the OID, this function will throw an error.
     A <structname>UserMapping</structname> object contains properties of the
     user mapping (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>GetForeignPaths</>, <function>GetForeignPlan</>,
     <function>PlanForeignModify</>, and <function>GetForeignJoinPaths</>
     must fit into the workings of the <productname>PostgreSQL</> planner.
     Here are some notes about what they must do.
    </para>

    <para>
     The information in <literal>root</> and <literal>baserel</> 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-&gt;baserestrictinfo</> is particularly interesting, as
     it contains restriction quals (<literal>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.)
     <literal>baserel-&gt;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 <structname>ForeignScan</> 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-&gt;fdw_private</> is a <type>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 <literal>RelOptInfo</> node is created.
     It is useful for passing information forward from
     <function>GetForeignRelSize</> to <function>GetForeignPaths</> and/or
     <function>GetForeignPaths</> to <function>GetForeignPlan</>, thereby
     avoiding recalculation.
    </para>

    <para>
     <function>GetForeignPaths</> can identify the meaning of different
     access paths by storing private information in the
     <structfield>fdw_private</> field of <structname>ForeignPath</> nodes.
     <structfield>fdw_private</> is declared as a <type>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 <function>nodeToString</>, for use with debugging support available
     in the backend.
    </para>

    <para>
     <function>GetForeignPlan</> can examine the <structfield>fdw_private</>
     field of the selected <structname>ForeignPath</> node, and can generate
     <structfield>fdw_exprs</> and <structfield>fdw_private</> lists to be
     placed in the <structname>ForeignScan</> plan node, where they will be
     available at execution time.  Both of these lists must be
     represented in a form that <function>copyObject</> knows how to copy.
     The <structfield>fdw_private</> list has no other restrictions and is
     not interpreted by the core backend in any way.  The
     <structfield>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.
    </para>

    <para>
     In <function>GetForeignPlan</>, generally the passed-in target list can
     be copied into the plan node as-is.  The passed <literal>scan_clauses</> list
     contains the same clauses as <literal>baserel-&gt;baserestrictinfo</>,
     but may be re-ordered for better execution efficiency.  In simple cases
     the FDW can just strip <structname>RestrictInfo</> nodes from the
     <literal>scan_clauses</> list (using <function>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.
    </para>

    <para>
     As an example, the FDW might identify some restriction clauses of the
     form <replaceable>foreign_variable</> <literal>=</>
     <replaceable>sub_expression</>, which it determines can be executed on
     the remote server given the locally-evaluated value of the
     <replaceable>sub_expression</>.  The actual identification of such a
     clause should happen during <function>GetForeignPaths</>, since it would
     affect the cost estimate for the path.  The path's
     <structfield>fdw_private</> field would probably include a pointer to
     the identified clause's <structname>RestrictInfo</> node.  Then
     <function>GetForeignPlan</> would remove that clause from <literal>scan_clauses</>,
     but add the <replaceable>sub_expression</> to <structfield>fdw_exprs</>
     to ensure that it gets massaged into executable form.  It would probably
     also put control information into the plan node's
     <structfield>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 <literal>WHERE <replaceable>foreign_variable</> =
     $1</literal>, with the parameter value obtained at run time from
     evaluation of the <structfield>fdw_exprs</> expression tree.
    </para>

    <para>
     Any clauses removed from the plan node's qual list must instead be added
     to <literal>fdw_recheck_quals</> or rechecked by
     <literal>RecheckForeignScan</> in order to ensure correct behavior
     at the <literal>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
     <literal>fdw_recheck_quals</> is typically easier than implementing checks
     inside <literal>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.
    </para>

    <para>
     Another <structname>ForeignScan</> field that can be filled by FDWs
     is <structfield>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 <literal>NIL</>, 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</>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</>.  Note that join
     plans (created from paths made by <function>GetForeignJoinPaths</>) must
     always supply <structfield>fdw_scan_tlist</> 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</> <literal>=</>
     <replaceable>local_variable</>.  Such clauses will not be found in
     <literal>baserel-&gt;baserestrictinfo</> but must be sought in the
     relation's join lists.  A path using such a clause is called a
     <quote>parameterized path</>.  It must identify the other relations
     used in the selected join clause(s) with a suitable value of
     <literal>param_info</>; use <function>get_baserel_parampathinfo</>
     to compute that value.  In <function>GetForeignPlan</>, the
     <replaceable>local_variable</> portion of the join clause would be added
     to <structfield>fdw_exprs</>, 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</> should
     produce <structname>ForeignPath</>s for potential remote joins in much
     the same way as <function>GetForeignPaths</> works for base tables.
     Information about the intended join can be passed forward
     to <function>GetForeignPlan</> in the same ways described above.
     However, <structfield>baserestrictinfo</> is not relevant for join
     relations; instead, the relevant join clauses for a particular join are
     passed to <function>GetForeignJoinPaths</> as a separate parameter
     (<literal>extra-&gt;restrictlist</>).
    </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
     (probably ForeignPaths or CustomPaths) and insert them into the
     appropriate <firstterm>upper relation</>.  For example, a path
     representing remote aggregation should be inserted into the relation
     obtained from <literal>fetch_upper_rel(root, UPPERREL_GROUP_AGG,
     NULL)</>, using <function>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 <function>GetForeignPlan</>.
    </para>

    <para>
     <function>PlanForeignModify</> 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</>
     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</> upper relation, where it would
     compete against the <literal>ModifyTable</> approach.  This approach
     could also be used to implement remote <literal>SELECT FOR UPDATE</>,
     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</> is responsible for
     implementing <emphasis>all</> behavior of the query.
    </para>

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

    <para>
     <command>INSERT</> with an <literal>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 <literal>ON CONFLICT DO UPDATE</> 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</> tables.  There are multiple
     considerations involved in this.
    </para>

    <para>
     One key decision to be made is whether to perform <firstterm>early
     locking</> or <firstterm>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 <productname>PostgreSQL</> TIDs do.
    </para>

    <para>
     By default, <productname>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 <xref linkend="fdw-callbacks-row-locking">, which were added
     in <productname>PostgreSQL</> 9.5, allow an FDW to use late locking if
     it wishes.
    </para>

    <para>
     An additional consideration is that in <literal>READ COMMITTED</>
     isolation mode, <productname>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 <productname>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.
    </para>

    <para>
     For an <command>UPDATE</> or <command>DELETE</> on a foreign table, it
     is recommended that the <literal>ForeignScan</> operation on the target
     table perform early locking on the rows that it fetches, perhaps via the
     equivalent of <command>SELECT FOR UPDATE</>.  An FDW can detect whether
     a table is an <command>UPDATE</>/<command>DELETE</> target at plan time
     by comparing its relid to <literal>root-&gt;parse-&gt;resultRelation</>,
     or at execution time by using <function>ExecRelationIsTargetRelation()</>.
     An alternative possibility is to perform late locking within the
     <function>ExecForeignUpdate</> or <function>ExecForeignDelete</>
     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, the <literal>ForeignScan</> operation can
     again perform early locking by fetching tuples with the equivalent
     of <command>SELECT FOR UPDATE/SHARE</>.  To perform late locking
     instead, provide the callback functions defined
     in <xref linkend="fdw-callbacks-row-locking">.
     In <function>GetForeignRowMarkType</>, select rowmark option
     <literal>ROW_MARK_EXCLUSIVE</>, <literal>ROW_MARK_NOKEYEXCLUSIVE</>,
     <literal>ROW_MARK_SHARE</>, or <literal>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 <function>get_plan_rowmark</> at
     plan time, or <function>ExecFindRowMark</> at execution time; you must
     check not only whether a non-null rowmark struct is returned, but that
     its <structfield>strength</> field is not <literal>LCS_NONE</>.
    </para>

    <para>
     Lastly, for foreign tables that are used in an <command>UPDATE</>,
     <command>DELETE</> or <command>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 <function>GetForeignRowMarkType</> select
     option <literal>ROW_MARK_REFERENCE</> when it sees lock strength
     <literal>LCS_NONE</>.  This will cause <function>RefetchForeignRow</> to
     be called with that value for <structfield>markType</>; it should then
     re-fetch the row without acquiring any new lock.  (If you have
     a <function>GetForeignRowMarkType</> function but don't wish to re-fetch
     unlocked rows, select option <literal>ROW_MARK_COPY</>
     for <literal>LCS_NONE</>.)
    </para>

    <para>
     See <filename>src/include/nodes/lockoptions.h</>, the comments
     for <type>RowMarkType</> and <type>PlanRowMark</>
     in <filename>src/include/nodes/plannodes.h</>, and the comments for
     <type>ExecRowMark</> in <filename>src/include/nodes/execnodes.h</> for
     additional information.
    </para>

  </sect1>

 </chapter>
