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Implement an API to let foreign-data wrappers actually be functional.

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This commit provides the core code and documentation needed.  A contrib
module test case will follow shortly.

Shigeru Hanada, Jan Urbanski, Heikki Linnakangas
This commit is contained in:
Tom Lane
2011-02-20 00:17:18 -05:00
parent d5813488a4
commit bb74240794
39 changed files with 1202 additions and 62 deletions
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47
doc/src/sgml/ddl.sgml
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@ -2986,6 +2986,53 @@ ANALYZE measurement;
</sect2>
</sect1>
<sect1 id="ddl-foreign-data">
<title>Foreign Data</title>
<indexterm>
<primary>foreign data</primary>
</indexterm>
<indexterm>
<primary>foreign table</primary>
</indexterm>
<para>
<productname>PostgreSQL</productname> implements portions of the SQL/MED
specification, allowing you to access data that resides outside
PostgreSQL using regular SQL queries. Such data is referred to as
<firstterm>foreign data</>. (Note that this usage is not to be confused
with foreign keys, which are a type of constraint within the database.)
</para>
<para>
Foreign data is accessed with help from a
<firstterm>foreign data wrapper</firstterm>. A foreign data wrapper is a
library that can communicate with an external data source, hiding the
details of connecting to the data source and fetching data from it. There
are several foreign data wrappers available, which can for example read
plain data files residing on the server, or connect to another PostgreSQL
instance. If none of the existing foreign data wrappers suit your needs,
you can write your own; see <xref linkend="fdwhandler">.
</para>
<para>
To access foreign data, you need to create a <firstterm>foreign server</>
object, which defines how to connect to a particular external data source,
according to the set of options used by a particular foreign data
wrapper. Then you need to create one or more <firstterm>foreign
tables</firstterm>, which define the structure of the remote data. A
foreign table can be used in queries just like a normal table, but a
foreign table has no storage in the PostgreSQL server. Whenever it is
used, PostgreSQL asks the foreign data wrapper to fetch the data from the
external source.
</para>
<para>
Currently, foreign tables are read-only. This limitation may be fixed
in a future release.
</para>
</sect1>
<sect1 id="ddl-others">
<title>Other Database Objects</title>

212
doc/src/sgml/fdwhandler.sgml Normal file
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@ -0,0 +1,212 @@
<!-- 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 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.
</para>
<para>
The validator function is responsible for validating options given in the
<command>CREATE FOREIGN DATA WRAPPER</command>, <command>CREATE
SERVER</command> and <command>CREATE FOREIGN TABLE</command> commands.
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). If no validator
function is supplied, the options are not checked at object creation time.
</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-routines">
<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>
FdwPlan *
PlanForeignScan (Oid foreigntableid,
PlannerInfo *root,
RelOptInfo *baserel);
</programlisting>
Plan 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.
The function must return a palloc'd struct that contains cost estimates
plus 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.
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 a pointer to the
<structname>FdwPlan</> structure returned 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>
<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</> and <structname>FdwPlan</> struct types
are declared in <filename>src/include/foreign/fdwapi.h</>, which see
for additional details.
</para>
</sect1>
</chapter>

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doc/src/sgml/filelist.sgml
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@ -86,6 +86,7 @@
<!entity indexam SYSTEM "indexam.sgml">
<!entity nls SYSTEM "nls.sgml">
<!entity plhandler SYSTEM "plhandler.sgml">
<!entity fdwhandler SYSTEM "fdwhandler.sgml">
<!entity protocol SYSTEM "protocol.sgml">
<!entity sources SYSTEM "sources.sgml">
<!entity storage SYSTEM "storage.sgml">

1
doc/src/sgml/postgres.sgml
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@ -238,6 +238,7 @@
&sources;
&nls;
&plhandler;
&fdwhandler;
&geqo;
&indexam;
&gist;

13
doc/src/sgml/ref/create_foreign_data_wrapper.sgml
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@ -119,18 +119,13 @@ CREATE FOREIGN DATA WRAPPER <replaceable class="parameter">name</replaceable>
<title>Notes</title>
<para>
At the moment, the foreign-data wrapper functionality is very
rudimentary. The purpose of foreign-data wrappers, foreign
servers, and user mappings is to store this information in a
standard way so that it can be queried by interested applications.
One such application is <application>dblink</application>;
see <xref linkend="dblink">. The functionality to actually query
external data through a foreign-data wrapper library does not exist
yet.
At the moment, the foreign-data wrapper functionality is rudimentary.
There is no support for updating a foreign table, and optimization of
queries is primitive (and mostly left to the wrapper, too).
</para>
<para>
There is currently one foreign-data wrapper validator function
There is one built-in foreign-data wrapper validator function
provided:
<filename>postgresql_fdw_validator</filename>, which accepts
options corresponding to <application>libpq</> connection

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doc/src/sgml/ref/create_foreign_table.sgml
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@ -131,8 +131,8 @@ CREATE FOREIGN TABLE [ IF NOT EXISTS ] <replaceable class="PARAMETER">table_name
<para>
Options to be associated with the new foreign table.
The allowed option names and values are specific to each foreign
data wrapper and are validated using the foreign-data wrapper
library. Option names must be unique.
data wrapper and are validated using the foreign-data wrapper's
validator function. Option names must be unique.
</para>
</listitem>
</varlistentry>