<!-- 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 planner and
    executor call. The foreign data wrapper is responsible for fetching
    data from the remote data source and returning it to the
    <productname>PostgreSQL</productname> executor. 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/file_fdw</> 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 and executor.
     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 following callback functions:
    </para>

    <para>
<programlisting>
void
PlanForeignScan (Oid foreigntableid,
                 PlannerInfo *root,
                 RelOptInfo *baserel);
</programlisting>

     Create possible access paths for a scan on a foreign table. This is
     called when a query is planned.
     <literal>foreigntableid</> is the <structname>pg_class</> OID of the
     foreign table.  <literal>root</> is the planner's global information
     about the query, and <literal>baserel</> is the planner's information
     about this table.
    </para>

    <para>
     The function must generate at least one access path (ForeignPath node)
     for a scan on the foreign table and must call <function>add_path</> to
     add the path to <literal>baserel-&gt;pathlist</>.  It's recommended to
     use <function>create_foreignscan_path</> to build the ForeignPath node.
     The function may 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 execute the foreign scan at a later time.
     (Note that the private information must be represented in a form that
     <function>copyObject</> knows how to copy.)
    </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 estimate).
     <literal>baserel-&gt;baserestrictinfo</> is particularly interesting, as
     it contains restriction quals (<literal>WHERE</> clauses) that can be
     used to filter the rows to be fetched.  (The FDW is not required to
     enforce these quals, as the finished plan will recheck them anyway.)
     <literal>baserel-&gt;reltargetlist</> can be used to determine which
     columns need to be fetched.
    </para>

    <para>
     In addition to returning cost estimates, the function should update
     <literal>baserel-&gt;rows</> to be the expected number of rows returned
     by the 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>
<programlisting>
void
ExplainForeignScan (ForeignScanState *node,
                    ExplainState *es);
</programlisting>

     Print additional <command>EXPLAIN</> output for a foreign table scan.
     This can just return if there is no need to print anything.
     Otherwise, it should 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 runtime statistics in the <command>EXPLAIN
     ANALYZE</> case.
    </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>PlanForeignScan</>).
    </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 column signature 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.
    </para>

    <para>
     Note that <productname>PostgreSQL</productname>'s executor doesn't care
     whether the rows returned violate the <literal>NOT NULL</literal>
     constraints which were defined on the foreign table columns - but the
     planner does care, and may optimize queries incorrectly if
     <literal>NULL</> values are present in a column declared not to contain
     them.  If a <literal>NULL</> value is encountered when the user has
     declared that none should be present, 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>

    <para>
     The <structname>FdwRoutine</> struct type is declared in
     <filename>src/include/foreign/fdwapi.h</>, which see for additional
     details.
    </para>

   </sect1>

 </chapter>
