Some mop-up work for savepoints (nested transactions). Store a small

number of active subtransaction XIDs in each backend's PGPROC entry,
and use this to avoid expensive probes into pg_subtrans during
TransactionIdIsInProgress.  Extend EOXactCallback API to allow add-on
modules to get control at subxact start/end.  (This is deliberately
not compatible with the former API, since any uses of that API probably
need manual review anyway.)  Add basic reference documentation for
SAVEPOINT and related commands.  Minor other cleanups to check off some
of the open issues for subtransactions.
Alvaro Herrera and Tom Lane.

doc/src/sgml/ref/rollback_to.sgml

@@ -0,0 +1,163 @@
+<!--
+$PostgreSQL: pgsql/doc/src/sgml/ref/rollback_to.sgml,v 1.1 2004/08/01 17:32:13 tgl Exp $
+PostgreSQL documentation
+-->
+
+<refentry id="SQL-ROLLBACK-TO">
+ <refmeta>
+  <refentrytitle id="SQL-ROLLBACK-TO-TITLE">ROLLBACK TO</refentrytitle>
+  <refmiscinfo>SQL - Language Statements</refmiscinfo>
+ </refmeta>
+
+ <refnamediv>
+  <refname>ROLLBACK TO</refname>
+  <refpurpose>roll back to a savepoint</refpurpose>
+ </refnamediv>
+
+ <indexterm zone="sql-rollback-to">
+  <primary>ROLLBACK TO</primary>
+ </indexterm>
+
+ <indexterm zone="sql-rollback-to">
+  <primary>savepoints</primary>
+  <secondary>rolling back</secondary>
+ </indexterm>
+
+ <refsynopsisdiv>
+<synopsis>
+ROLLBACK TO <replaceable>savepoint_name</replaceable>
+</synopsis>
+ </refsynopsisdiv>
+
+ <refsect1>
+  <title>Description</title>
+
+  <para>
+   Roll back all commands that were executed after the savepoint was
+   established.  The savepoint remains valid and can be rolled back to
+   again later, if needed.
+  </para>
+
+  <para>
+   <command>ROLLBACK TO</> implicitly destroys all savepoints that
+   were established after the named savepoint.
+  </para>
+ </refsect1>
+
+ <refsect1>
+  <title>Parameters</title>
+
+  <variablelist>
+   <varlistentry>
+    <term><replaceable class="PARAMETER">savepoint_name</></term>
+    <listitem>
+     <para>
+      The savepoint to roll back to.
+     </para>
+    </listitem>
+   </varlistentry>
+  </variablelist>
+ </refsect1>
+
+ <refsect1>
+  <title>Notes</title>
+
+  <para>
+   Use <xref linkend="SQL-RELEASE" endterm="SQL-RELEASE-TITLE"> to
+   destroy a savepoint without discarding the effects of commands executed
+   after it was established.
+  </para>
+
+  <para>
+   Specifying a savepoint name that has not been established is an error.
+  </para>
+
+  <para>
+   Cursors have somewhat non-transactional behavior with respect to
+   savepoints.  Any cursor that is opened inside the savepoint is not closed
+   when the savepoint is rolled back.  If a cursor is affected by a
+   <command>FETCH</> command inside a savepoint that is later rolled
+   back, the cursor position remains at the position that <command>FETCH</>
+   left it pointing to (that is, <command>FETCH</> is not rolled back).
+   A cursor whose execution causes a transaction to abort is put in a
+   can't-execute state, so while the transaction can be restored using
+   <command>ROLLBACK TO</>, the cursor can no longer be used.
+  </para>
+ </refsect1>
+
+ <refsect1>
+  <title>Examples</title>
+
+  <para>
+   To undo the effects of the commands executed after <literal>my_savepoint</literal>
+   was established:
+<programlisting>
+ROLLBACK TO my_savepoint;
+</programlisting>
+  </para>
+
+  <para>
+   Cursor positions are not affected by savepoint rollback:
+<programlisting>
+BEGIN;
+
+DECLARE foo CURSOR FOR SELECT 1 UNION SELECT 2;
+
+SAVEPOINT foo;
+
+FETCH 1 FROM foo;
+ ?column? 
+----------
+        1
+
+ROLLBACK TO foo;
+
+FETCH 1 FROM foo;
+ ?column? 
+----------
+        2
+
+COMMIT;
+</programlisting>
+   </para>
+
+
+ </refsect1>
+
+ <refsect1>
+  <title>Compatibility</title>
+
+  <para>
+   This command is fully SQL standard conforming.
+  </para>
+ </refsect1>
+
+ <refsect1>
+  <title>See Also</title>
+
+  <simplelist type="inline">
+   <member><xref linkend="sql-begin" endterm="sql-begin-title"></member>
+   <member><xref linkend="sql-commit" endterm="sql-commit-title"></member>
+   <member><xref linkend="sql-savepoint" endterm="sql-savepoint-title"></member>
+   <member><xref linkend="sql-release" endterm="sql-release-title"></member>
+   <member><xref linkend="sql-rollback" endterm="sql-rollback-title"></member>
+  </simplelist>
+ </refsect1>
+</refentry>
+
+<!-- Keep this comment at the end of the file
+Local variables:
+mode: sgml
+sgml-omittag:nil
+sgml-shorttag:t
+sgml-minimize-attributes:nil
+sgml-always-quote-attributes:t
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+sgml-local-ecat-files:nil
+End:
+-->