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dc3eb56383
Extend the FDW API (which we already changed for 9.3) so that an FDW can report whether specific foreign tables are insertable/updatable/deletable. The default assumption continues to be that they're updatable if the relevant executor callback function is supplied by the FDW, but finer granularity is now possible. As a test case, add an "updatable" option to contrib/postgres_fdw. This patch also fixes the information_schema views, which previously did not think that foreign tables were ever updatable, and fixes view_is_auto_updatable() so that a view on a foreign table can be auto-updatable. initdb forced due to changes in information_schema views and the functions they rely on. This is a bit unfortunate to do post-beta1, but if we don't change this now then we'll have another API break for FDWs when we do change it. Dean Rasheed, somewhat editorialized on by Tom Lane
956 lines
39 KiB
Plaintext
956 lines
39 KiB
Plaintext
<!-- doc/src/sgml/fdwhandler.sgml -->
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<chapter id="fdwhandler">
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<title>Writing A Foreign Data Wrapper</title>
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<indexterm zone="fdwhandler">
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<primary>foreign data wrapper</primary>
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<secondary>handler for</secondary>
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</indexterm>
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<para>
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All operations on a foreign table are handled through its foreign data
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wrapper, which consists of a set of functions that the core server
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calls. The foreign data wrapper is responsible for fetching
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data from the remote data source and returning it to the
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<productname>PostgreSQL</productname> executor. If updating foreign
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tables is to be supported, the wrapper must handle that, too.
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This chapter outlines how to write a new foreign data wrapper.
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</para>
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<para>
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The foreign data wrappers included in the standard distribution are good
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references when trying to write your own. Look into the
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<filename>contrib</> subdirectory of the source tree.
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The <xref linkend="sql-createforeigndatawrapper"> reference page also has
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some useful details.
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</para>
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<note>
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<para>
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The SQL standard specifies an interface for writing foreign data wrappers.
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However, PostgreSQL does not implement that API, because the effort to
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accommodate it into PostgreSQL would be large, and the standard API hasn't
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gained wide adoption anyway.
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</para>
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</note>
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<sect1 id="fdw-functions">
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<title>Foreign Data Wrapper Functions</title>
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<para>
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The FDW author needs to implement a handler function, and optionally
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a validator function. Both functions must be written in a compiled
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language such as C, using the version-1 interface.
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For details on C language calling conventions and dynamic loading,
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see <xref linkend="xfunc-c">.
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</para>
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<para>
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The handler function simply returns a struct of function pointers to
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callback functions that will be called by the planner, executor, and
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various maintenance commands.
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Most of the effort in writing an FDW is in implementing these callback
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functions.
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The handler function must be registered with
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<productname>PostgreSQL</productname> as taking no arguments and
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returning the special pseudo-type <type>fdw_handler</type>. The
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callback functions are plain C functions and are not visible or
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callable at the SQL level. The callback functions are described in
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<xref linkend="fdw-callbacks">.
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</para>
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<para>
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The validator function is responsible for validating options given in
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<command>CREATE</command> and <command>ALTER</command> commands for its
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foreign data wrapper, as well as foreign servers, user mappings, and
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foreign tables using the wrapper.
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The validator function must be registered as taking two arguments, a
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text array containing the options to be validated, and an OID
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representing the type of object the options are associated with (in
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the form of the OID of the system catalog the object would be stored
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in, either
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<literal>ForeignDataWrapperRelationId</>,
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<literal>ForeignServerRelationId</>,
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<literal>UserMappingRelationId</>,
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or <literal>ForeignTableRelationId</>).
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If no validator function is supplied, options are not checked at object
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creation time or object alteration time.
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</para>
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</sect1>
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<sect1 id="fdw-callbacks">
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<title>Foreign Data Wrapper Callback Routines</title>
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<para>
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The FDW handler function returns a palloc'd <structname>FdwRoutine</>
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struct containing pointers to the callback functions described below.
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The scan-related functions are required, the rest are optional.
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</para>
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<para>
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The <structname>FdwRoutine</> struct type is declared in
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<filename>src/include/foreign/fdwapi.h</>, which see for additional
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details.
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</para>
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<sect2 id="fdw-callbacks-scan">
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<title>FDW Routines For Scanning Foreign Tables</title>
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<para>
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<programlisting>
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void
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GetForeignRelSize (PlannerInfo *root,
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RelOptInfo *baserel,
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Oid foreigntableid);
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</programlisting>
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Obtain relation size estimates for a foreign table. This is called
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at the beginning of planning for a query that scans a foreign table.
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<literal>root</> is the planner's global information about the query;
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<literal>baserel</> is the planner's information about this table; and
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<literal>foreigntableid</> is the <structname>pg_class</> OID of the
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foreign table. (<literal>foreigntableid</> could be obtained from the
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planner data structures, but it's passed explicitly to save effort.)
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</para>
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<para>
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This function should update <literal>baserel->rows</> to be the
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expected number of rows returned by the table scan, after accounting for
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the filtering done by the restriction quals. The initial value of
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<literal>baserel->rows</> is just a constant default estimate, which
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should be replaced if at all possible. The function may also choose to
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update <literal>baserel->width</> if it can compute a better estimate
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of the average result row width.
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</para>
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<para>
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See <xref linkend="fdw-planning"> for additional information.
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</para>
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<para>
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<programlisting>
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void
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GetForeignPaths (PlannerInfo *root,
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RelOptInfo *baserel,
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Oid foreigntableid);
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</programlisting>
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Create possible access paths for a scan on a foreign table.
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This is called during query planning.
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The parameters are the same as for <function>GetForeignRelSize</>,
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which has already been called.
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</para>
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<para>
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This function must generate at least one access path
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(<structname>ForeignPath</> node) for a scan on the foreign table and
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must call <function>add_path</> to add each such path to
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<literal>baserel->pathlist</>. It's recommended to use
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<function>create_foreignscan_path</> to build the
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<structname>ForeignPath</> nodes. The function can generate multiple
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access paths, e.g., a path which has valid <literal>pathkeys</> to
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represent a pre-sorted result. Each access path must contain cost
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estimates, and can contain any FDW-private information that is needed to
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identify the specific scan method intended.
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</para>
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<para>
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See <xref linkend="fdw-planning"> for additional information.
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</para>
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<para>
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<programlisting>
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ForeignScan *
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GetForeignPlan (PlannerInfo *root,
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RelOptInfo *baserel,
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Oid foreigntableid,
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ForeignPath *best_path,
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List *tlist,
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List *scan_clauses);
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</programlisting>
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Create a <structname>ForeignScan</> plan node from the selected foreign
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access path. This is called at the end of query planning.
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The parameters are as for <function>GetForeignRelSize</>, plus
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the selected <structname>ForeignPath</> (previously produced by
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<function>GetForeignPaths</>), the target list to be emitted by the
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plan node, and the restriction clauses to be enforced by the plan node.
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</para>
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<para>
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This function must create and return a <structname>ForeignScan</> plan
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node; it's recommended to use <function>make_foreignscan</> to build the
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<structname>ForeignScan</> node.
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</para>
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<para>
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See <xref linkend="fdw-planning"> for additional information.
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</para>
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<para>
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<programlisting>
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void
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BeginForeignScan (ForeignScanState *node,
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int eflags);
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</programlisting>
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Begin executing a foreign scan. This is called during executor startup.
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It should perform any initialization needed before the scan can start,
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but not start executing the actual scan (that should be done upon the
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first call to <function>IterateForeignScan</>).
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The <structname>ForeignScanState</> node has already been created, but
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its <structfield>fdw_state</> field is still NULL. Information about
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the table to scan is accessible through the
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<structname>ForeignScanState</> node (in particular, from the underlying
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<structname>ForeignScan</> plan node, which contains any FDW-private
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information provided by <function>GetForeignPlan</>).
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<literal>eflags</> contains flag bits describing the executor's
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operating mode for this plan node.
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</para>
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<para>
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Note that when <literal>(eflags & EXEC_FLAG_EXPLAIN_ONLY)</> is
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true, this function should not perform any externally-visible actions;
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it should only do the minimum required to make the node state valid
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for <function>ExplainForeignScan</> and <function>EndForeignScan</>.
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</para>
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<para>
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<programlisting>
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TupleTableSlot *
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IterateForeignScan (ForeignScanState *node);
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</programlisting>
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Fetch one row from the foreign source, returning it in a tuple table slot
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(the node's <structfield>ScanTupleSlot</> should be used for this
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purpose). Return NULL if no more rows are available. The tuple table
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slot infrastructure allows either a physical or virtual tuple to be
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returned; in most cases the latter choice is preferable from a
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performance standpoint. Note that this is called in a short-lived memory
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context that will be reset between invocations. Create a memory context
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in <function>BeginForeignScan</> if you need longer-lived storage, or use
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the <structfield>es_query_cxt</> of the node's <structname>EState</>.
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</para>
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<para>
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The rows returned must match the column signature of the foreign table
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being scanned. If you choose to optimize away fetching columns that
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are not needed, you should insert nulls in those column positions.
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</para>
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<para>
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Note that <productname>PostgreSQL</productname>'s executor doesn't care
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whether the rows returned violate any <literal>NOT NULL</literal>
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constraints that were defined on the foreign table columns — but
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the planner does care, and may optimize queries incorrectly if
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<literal>NULL</> values are present in a column declared not to contain
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them. If a <literal>NULL</> value is encountered when the user has
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declared that none should be present, it may be appropriate to raise an
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error (just as you would need to do in the case of a data type mismatch).
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</para>
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<para>
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<programlisting>
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void
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ReScanForeignScan (ForeignScanState *node);
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</programlisting>
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Restart the scan from the beginning. Note that any parameters the
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scan depends on may have changed value, so the new scan does not
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necessarily return exactly the same rows.
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</para>
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<para>
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<programlisting>
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void
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EndForeignScan (ForeignScanState *node);
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</programlisting>
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End the scan and release resources. It is normally not important
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to release palloc'd memory, but for example open files and connections
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to remote servers should be cleaned up.
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</para>
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</sect2>
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<sect2 id="fdw-callbacks-update">
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<title>FDW Routines For Updating Foreign Tables</title>
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<para>
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If an FDW supports writable foreign tables, it should provide
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some or all of the following callback functions depending on
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the needs and capabilities of the FDW:
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</para>
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<para>
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<programlisting>
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void
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AddForeignUpdateTargets (Query *parsetree,
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RangeTblEntry *target_rte,
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Relation target_relation);
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</programlisting>
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<command>UPDATE</> and <command>DELETE</> operations are performed
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against rows previously fetched by the table-scanning functions. The
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FDW may need extra information, such as a row ID or the values of
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primary-key columns, to ensure that it can identify the exact row to
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update or delete. To support that, this function can add extra hidden,
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or <quote>junk</>, target columns to the list of columns that are to be
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retrieved from the foreign table during an <command>UPDATE</> or
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<command>DELETE</>.
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</para>
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<para>
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To do that, add <structname>TargetEntry</> items to
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<literal>parsetree->targetList</>, containing expressions for the
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extra values to be fetched. Each such entry must be marked
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<structfield>resjunk</> = <literal>true</>, and must have a distinct
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<structfield>resname</> that will identify it at execution time.
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Avoid using names matching <literal>ctid<replaceable>N</></literal> or
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<literal>wholerow<replaceable>N</></literal>, as the core system can
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generate junk columns of these names.
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</para>
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<para>
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This function is called in the rewriter, not the planner, so the
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information available is a bit different from that available to the
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planning routines.
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<literal>parsetree</> is the parse tree for the <command>UPDATE</> or
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<command>DELETE</> command, while <literal>target_rte</> and
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<literal>target_relation</> describe the target foreign table.
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</para>
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<para>
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If the <function>AddForeignUpdateTargets</> pointer is set to
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<literal>NULL</>, no extra target expressions are added.
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(This will make it impossible to implement <command>DELETE</>
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operations, though <command>UPDATE</> may still be feasible if the FDW
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relies on an unchanging primary key to identify rows.)
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</para>
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<para>
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<programlisting>
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List *
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PlanForeignModify (PlannerInfo *root,
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ModifyTable *plan,
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Index resultRelation,
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int subplan_index);
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</programlisting>
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Perform any additional planning actions needed for an insert, update, or
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delete on a foreign table. This function generates the FDW-private
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information that will be attached to the <structname>ModifyTable</> plan
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node that performs the update action. This private information must
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have the form of a <literal>List</>, and will be delivered to
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<function>BeginForeignModify</> during the execution stage.
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</para>
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<para>
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<literal>root</> is the planner's global information about the query.
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<literal>plan</> is the <structname>ModifyTable</> plan node, which is
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complete except for the <structfield>fdwPrivLists</> field.
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<literal>resultRelation</> identifies the target foreign table by its
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rangetable index. <literal>subplan_index</> identifies which target of
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the <structname>ModifyTable</> plan node this is, counting from zero;
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use this if you want to index into <literal>plan->plans</> or other
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substructure of the <literal>plan</> node.
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</para>
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<para>
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See <xref linkend="fdw-planning"> for additional information.
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</para>
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<para>
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If the <function>PlanForeignModify</> pointer is set to
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<literal>NULL</>, no additional plan-time actions are taken, and the
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<literal>fdw_private</> list delivered to
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<function>BeginForeignModify</> will be NIL.
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</para>
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<para>
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<programlisting>
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void
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BeginForeignModify (ModifyTableState *mtstate,
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ResultRelInfo *rinfo,
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List *fdw_private,
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int subplan_index,
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int eflags);
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</programlisting>
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Begin executing a foreign table modification operation. This routine is
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called during executor startup. It should perform any initialization
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needed prior to the actual table modifications. Subsequently,
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<function>ExecForeignInsert</>, <function>ExecForeignUpdate</> or
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<function>ExecForeignDelete</> will be called for each tuple to be
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inserted, updated, or deleted.
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</para>
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<para>
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<literal>mtstate</> is the overall state of the
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<structname>ModifyTable</> plan node being executed; global data about
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the plan and execution state is available via this structure.
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<literal>rinfo</> is the <structname>ResultRelInfo</> struct describing
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the target foreign table. (The <structfield>ri_FdwState</> field of
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<structname>ResultRelInfo</> is available for the FDW to store any
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private state it needs for this operation.)
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<literal>fdw_private</> contains the private data generated by
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<function>PlanForeignModify</>, if any.
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<literal>subplan_index</> identifies which target of
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the <structname>ModifyTable</> plan node this is.
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<literal>eflags</> contains flag bits describing the executor's
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operating mode for this plan node.
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</para>
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<para>
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Note that when <literal>(eflags & EXEC_FLAG_EXPLAIN_ONLY)</> is
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true, this function should not perform any externally-visible actions;
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it should only do the minimum required to make the node state valid
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for <function>ExplainForeignModify</> and <function>EndForeignModify</>.
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</para>
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<para>
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If the <function>BeginForeignModify</> pointer is set to
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<literal>NULL</>, no action is taken during executor startup.
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</para>
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<para>
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<programlisting>
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TupleTableSlot *
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ExecForeignInsert (EState *estate,
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ResultRelInfo *rinfo,
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TupleTableSlot *slot,
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TupleTableSlot *planSlot);
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</programlisting>
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Insert one tuple into the foreign table.
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<literal>estate</> is global execution state for the query.
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<literal>rinfo</> is the <structname>ResultRelInfo</> struct describing
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the target foreign table.
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<literal>slot</> contains the tuple to be inserted; it will match the
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rowtype definition of the foreign table.
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<literal>planSlot</> contains the tuple that was generated by the
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<structname>ModifyTable</> plan node's subplan; it differs from
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<literal>slot</> in possibly containing additional <quote>junk</>
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columns. (The <literal>planSlot</> is typically of little interest
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for <command>INSERT</> cases, but is provided for completeness.)
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</para>
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<para>
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The return value is either a slot containing the data that was actually
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inserted (this might differ from the data supplied, for example as a
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result of trigger actions), or NULL if no row was actually inserted
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(again, typically as a result of triggers). The passed-in
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<literal>slot</> can be re-used for this purpose.
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</para>
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<para>
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The data in the returned slot is used only if the <command>INSERT</>
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query has a <literal>RETURNING</> clause. Hence, the FDW could choose
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to optimize away returning some or all columns depending on the contents
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of the <literal>RETURNING</> clause. However, some slot must be
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returned to indicate success, or the query's reported row count will be
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wrong.
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</para>
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<para>
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If the <function>ExecForeignInsert</> pointer is set to
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<literal>NULL</>, attempts to insert into the foreign table will fail
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with an error message.
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</para>
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<para>
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<programlisting>
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TupleTableSlot *
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ExecForeignUpdate (EState *estate,
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ResultRelInfo *rinfo,
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TupleTableSlot *slot,
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TupleTableSlot *planSlot);
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</programlisting>
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Update one tuple in the foreign table.
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<literal>estate</> is global execution state for the query.
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<literal>rinfo</> is the <structname>ResultRelInfo</> struct describing
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the target foreign table.
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<literal>slot</> contains the new data for the tuple; it will match the
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rowtype definition of the foreign table.
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<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. Hence, the FDW could choose
|
|
to optimize away returning some or all columns depending on the contents
|
|
of the <literal>RETURNING</> clause. However, 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. Hence, the FDW could choose
|
|
to optimize away returning some or all columns depending on the contents
|
|
of the <literal>RETURNING</> clause. However, 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 bitmask 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-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>
|
|
|
|
</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>
|
|
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</>, and
|
|
<function>PlanForeignModify</> 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->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->reltargetlist</> 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->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>baserel</> 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 scan_clauses list
|
|
contains the same clauses as <literal>baserel->baserestrictinfo</>,
|
|
but may be re-ordered for better execution efficiency. In simple cases
|
|
the FDW can just strip <structname>RestrictInfo</> nodes from the
|
|
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 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>
|
|
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->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>
|
|
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->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>
|
|
For an <command>UPDATE</> or <command>DELETE</> against an external data
|
|
source that supports concurrent updates, it is recommended that the
|
|
<literal>ForeignScan</> operation lock the rows that it fetches, perhaps
|
|
via the equivalent of <command>SELECT FOR UPDATE</>. The FDW may also
|
|
choose to lock rows at fetch time when the foreign table is referenced
|
|
in a <command>SELECT FOR UPDATE/SHARE</>; if it does not, the
|
|
<literal>FOR UPDATE</> or <literal>FOR SHARE</> option is essentially a
|
|
no-op so far as the foreign table is concerned. This behavior may yield
|
|
semantics slightly different from operations on local tables, where row
|
|
locking is customarily delayed as long as possible: remote rows may get
|
|
locked even though they subsequently fail locally-applied restriction or
|
|
join conditions. However, matching the local semantics exactly would
|
|
require an additional remote access for every row, and might be
|
|
impossible anyway depending on what locking semantics the external data
|
|
source provides.
|
|
</para>
|
|
|
|
</sect1>
|
|
|
|
</chapter>
|