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Commitbb74240

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Implement an API to let foreign-data wrappers actually be functional.
This commit provides the core code and documentation needed. A contribmodule test case will follow shortly.Shigeru Hanada, Jan Urbanski, Heikki Linnakangas
1 parentd581348 commitbb74240

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‎doc/src/sgml/ddl.sgml

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</sect2>
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</sect1>
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<sect1 id="ddl-foreign-data">
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<title>Foreign Data</title>
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<indexterm>
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<primary>foreign data</primary>
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</indexterm>
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<indexterm>
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<primary>foreign table</primary>
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</indexterm>
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<para>
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<productname>PostgreSQL</productname> implements portions of the SQL/MED
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specification, allowing you to access data that resides outside
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PostgreSQL using regular SQL queries. Such data is referred to as
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<firstterm>foreign data</>. (Note that this usage is not to be confused
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with foreign keys, which are a type of constraint within the database.)
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</para>
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<para>
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Foreign data is accessed with help from a
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<firstterm>foreign data wrapper</firstterm>. A foreign data wrapper is a
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library that can communicate with an external data source, hiding the
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details of connecting to the data source and fetching data from it. There
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are several foreign data wrappers available, which can for example read
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plain data files residing on the server, or connect to another PostgreSQL
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instance. If none of the existing foreign data wrappers suit your needs,
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you can write your own; see <xref linkend="fdwhandler">.
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</para>
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<para>
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To access foreign data, you need to create a <firstterm>foreign server</>
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object, which defines how to connect to a particular external data source,
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according to the set of options used by a particular foreign data
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wrapper. Then you need to create one or more <firstterm>foreign
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tables</firstterm>, which define the structure of the remote data. A
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foreign table can be used in queries just like a normal table, but a
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foreign table has no storage in the PostgreSQL server. Whenever it is
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used, PostgreSQL asks the foreign data wrapper to fetch the data from the
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external source.
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</para>
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<para>
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Currently, foreign tables are read-only. This limitation may be fixed
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in a future release.
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</para>
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</sect1>
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<sect1 id="ddl-others">
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<title>Other Database Objects</title>
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‎doc/src/sgml/fdwhandler.sgml

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<!-- 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 planner and
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executor call. 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. This chapter outlines how
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to write a new foreign data wrapper.
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</para>
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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 and executor.
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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 returning
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the special pseudo-type <type>fdw_handler</type>.
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The callback functions are plain C functions and are not visible or
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callable at the SQL level.
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</para>
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<para>
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The validator function is responsible for validating options given in the
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<command>CREATE FOREIGN DATA WRAPPER</command>, <command>CREATE
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SERVER</command> and <command>CREATE FOREIGN TABLE</command> commands.
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The validator function must be registered as taking two arguments, a text
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array containing the options to be validated, and an OID representing the
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type of object the options are associated with (in the form of the OID
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of the system catalog the object would be stored in). If no validator
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function is supplied, the options are not checked at object creation time.
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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/file_fdw</> 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-routines">
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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 following callback functions:
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</para>
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<para>
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<programlisting>
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FdwPlan *
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PlanForeignScan (Oid foreigntableid,
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PlannerInfo *root,
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RelOptInfo *baserel);
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</programlisting>
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Plan a scan on a foreign table. This is called when a query is planned.
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<literal>foreigntableid</> is the <structname>pg_class</> OID of the
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foreign table. <literal>root</> is the planner's global information
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about the query, and <literal>baserel</> is the planner's information
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about this table.
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The function must return a palloc'd struct that contains cost estimates
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plus any FDW-private information that is needed to execute the foreign
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scan at a later time. (Note that the private information must be
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represented in a form that <function>copyObject</> knows how to copy.)
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</para>
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<para>
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The information in <literal>root</> and <literal>baserel</> can be used
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to reduce the amount of information that has to be fetched from the
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foreign table (and therefore reduce the cost estimate).
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<literal>baserel-&gt;baserestrictinfo</> is particularly interesting, as
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it contains restriction quals (<literal>WHERE</> clauses) that can be
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used to filter the rows to be fetched. (The FDW is not required to
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enforce these quals, as the finished plan will recheck them anyway.)
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<literal>baserel-&gt;reltargetlist</> can be used to determine which
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columns need to be fetched.
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</para>
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<para>
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In addition to returning cost estimates, the function should update
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<literal>baserel-&gt;rows</> to be the expected number of rows returned
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by the scan, after accounting for the filtering done by the restriction
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quals. The initial value of <literal>baserel-&gt;rows</> is just a
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constant default estimate, which should be replaced if at all possible.
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The function may also choose to update <literal>baserel-&gt;width</> if
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it can compute a better estimate of the average result row width.
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</para>
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<para>
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<programlisting>
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void
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ExplainForeignScan (ForeignScanState *node,
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ExplainState *es);
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</programlisting>
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Print additional <command>EXPLAIN</> output for a foreign table scan.
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This can just return if there is no need to print anything.
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Otherwise, it should call <function>ExplainPropertyText</> and
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related functions to add fields to the <command>EXPLAIN</> output.
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The flag fields in <literal>es</> can be used to determine what to
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print, and the state of the <structname>ForeignScanState</> node
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can be inspected to provide runtime statistics in the <command>EXPLAIN
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ANALYZE</> case.
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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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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 a pointer to the
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<structname>FdwPlan</> structure returned by
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<function>PlanForeignScan</>).
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</para>
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<para>
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Note that when <literal>(eflags &amp; 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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<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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<para>
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The <structname>FdwRoutine</> and <structname>FdwPlan</> struct types
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are declared in <filename>src/include/foreign/fdwapi.h</>, which see
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for additional details.
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</para>
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</sect1>
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</chapter>

‎doc/src/sgml/filelist.sgml

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<!entity indexam SYSTEM "indexam.sgml">
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<!entity nls SYSTEM "nls.sgml">
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<!entity plhandler SYSTEM "plhandler.sgml">
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<!entity fdwhandler SYSTEM "fdwhandler.sgml">
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<!entity protocol SYSTEM "protocol.sgml">
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<!entity sources SYSTEM "sources.sgml">
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<!entity storage SYSTEM "storage.sgml">

‎doc/src/sgml/postgres.sgml

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&sources;
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&nls;
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&plhandler;
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&fdwhandler;
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&geqo;
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&indexam;
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&gist;

‎doc/src/sgml/ref/create_foreign_data_wrapper.sgml

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<title>Notes</title>
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<para>
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At the moment, the foreign-data wrapper functionality is very
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rudimentary. The purpose of foreign-data wrappers, foreign
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servers, and user mappings is to store this information in a
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standard way so that it can be queried by interested applications.
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One such application is <application>dblink</application>;
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see <xref linkend="dblink">. The functionality to actually query
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external data through a foreign-data wrapper library does not exist
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yet.
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At the moment, the foreign-data wrapper functionality is rudimentary.
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There is no support for updating a foreign table, and optimization of
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queries is primitive (and mostly left to the wrapper, too).
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</para>
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<para>
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There iscurrentlyone foreign-data wrapper validator function
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There is one built-in foreign-data wrapper validator function
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provided:
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<filename>postgresql_fdw_validator</filename>, which accepts
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options corresponding to <application>libpq</> connection

‎doc/src/sgml/ref/create_foreign_table.sgml

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<para>
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Options to be associated with the new foreign table.
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The allowed option names and values are specific to each foreign
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data wrapper and are validated using the foreign-data wrapper
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library. Option names must be unique.
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data wrapper and are validated using the foreign-data wrapper's
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validator function. Option names must be unique.
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</para>
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</listitem>
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</varlistentry>

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