<!-- 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
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 involving 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);
</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</>), the target list to be emitted by the
     plan node, and the restriction clauses to be enforced by the plan node.
    </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
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>GetForeignPlan</>).
    </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>

   <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 *
GetForeignTableColumnOptions(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</>, and <function>GetForeignPlan</> 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;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-&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>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 runtime.  These
     trees will undergo post-processing by the planner to make them fully
     executable.
    </para>

    <para>
     In <function>GetForeignPlan</>, generally the passed-in targetlist can
     be copied into the plan node as-is.  The passed 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
     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 runtime.  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 runtime.  The query transmitted to the remote server would
     involve something like <literal>WHERE <replaceable>foreign_variable</> =
     $1</literal>, with the parameter value obtained at runtime 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-&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 show the other relation(s) as
     <literal>required_outer</> and list the specific join clause(s) in
     <literal>param_clauses</>.  In <function>GetForeignPlan</>, the
     <replaceable>local_variable</> portion of the join clause would be added
     to <structfield>fdw_exprs</>, and then at runtime the case works the
     same as for an ordinary restriction clause.
    </para>

  </sect1>

 </chapter>
