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

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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.
This commit is contained in:
Tom Lane
2004-08-01 17:32:22 +00:00
parent 9d9cdf82a4
commit efcaf1e868
23 changed files with 1103 additions and 330 deletions
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doc/src/sgml/ref/savepoint.sgml Normal file
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<!--
$PostgreSQL: pgsql/doc/src/sgml/ref/savepoint.sgml,v 1.1 2004/08/01 17:32:13 tgl Exp $
PostgreSQL documentation
-->
<refentry id="SQL-SAVEPOINT">
<refmeta>
<refentrytitle id="SQL-SAVEPOINT-TITLE">SAVEPOINT</refentrytitle>
<refmiscinfo>SQL - Language Statements</refmiscinfo>
</refmeta>
<refnamediv>
<refname>SAVEPOINT</refname>
<refpurpose>define a new savepoint within the current transaction</refpurpose>
</refnamediv>
<indexterm zone="sql-savepoint">
<primary>SAVEPOINT</primary>
</indexterm>
<indexterm zone="sql-savepoint">
<primary>savepoints</primary>
<secondary>defining</secondary>
</indexterm>
<refsynopsisdiv>
<synopsis>
SAVEPOINT <replaceable>savepoint_name</replaceable>
</synopsis>
</refsynopsisdiv>
<refsect1>
<title>Description</title>
<para>
<command>SAVEPOINT</command> establishes a new savepoint within
the current transaction.
</para>
<para>
A savepoint is a special mark inside a transaction that allows all commands
that are executed after it was established to be rolled back, restoring
the transaction state to what it was at the time of the savepoint.
</para>
</refsect1>
<refsect1>
<title>Parameters</title>
<variablelist>
<varlistentry>
<term><replaceable>savepoint_name</replaceable></term>
<listitem>
<para>
The name to give to the new savepoint.
</para>
</listitem>
</varlistentry>
</variablelist>
</refsect1>
<refsect1>
<title>Notes</title>
<para>
Use <xref linkend="SQL-ROLLBACK-TO" endterm="SQL-ROLLBACK-TO-TITLE"> to
rollback to a savepoint. Use <xref linkend="SQL-RELEASE"
endterm="SQL-RELEASE-TITLE"> to destroy a savepoint, keeping
the effects of commands executed after it was established.
</para>
<para>
Savepoints can only be established when inside a transaction block.
There can be multiple savepoints defined within a transaction.
</para>
</refsect1>
<refsect1>
<title>Examples</title>
<para>
To establish a savepoint and later undo the effects of all commands executed
after it was established:
<programlisting>
BEGIN;
INSERT INTO table VALUES (1);
SAVEPOINT my_savepoint;
INSERT INTO table VALUES (2);
ROLLBACK TO my_savepoint;
INSERT INTO table VALUES (3);
COMMIT;
</programlisting>
The above transaction will insert the values 1 and 3, but not 2.
</para>
<para>
To establish and later destroy a savepoint:
<programlisting>
BEGIN;
INSERT INTO table VALUES (3);
SAVEPOINT my_savepoint;
INSERT INTO table VALUES (4);
RELEASE my_savepoint;
COMMIT;
</programlisting>
The above transaction will insert both 3 and 4.
</para>
</refsect1>
<refsect1>
<title>Compatibility</title>
<para>
SQL requires a savepoint to be destroyed automatically when another
savepoint with the same name is established. In
<productname>PostgreSQL</>, the old savepoint is kept, though only the more
recent one will be used when rolling back or releasing. (Releasing the
newer savepoint will cause the older one to again become accessible to
<command>ROLLBACK TO</> and <command>RELEASE</>.)
Other than that, <command>SAVEPOINT</command> is fully SQL 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-rollback" endterm="sql-rollback-title"></member>
<member><xref linkend="sql-rollback-to" endterm="sql-rollback-to-title"></member>
<member><xref linkend="sql-release" endterm="sql-release-title"></member>
<member><xref linkend="sql-commit" endterm="sql-commit-title"></member>
</simplelist>
</refsect1>
</refentry>
<!-- Keep this comment at the end of the file
Local variables:
mode: sgml
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sgml-shorttag:t
sgml-minimize-attributes:nil
sgml-always-quote-attributes:t
sgml-indent-step:1
sgml-indent-data:t
sgml-parent-document:nil
sgml-default-dtd-file:"../reference.ced"
sgml-exposed-tags:nil
sgml-local-catalogs:"/usr/lib/sgml/catalog"
sgml-local-ecat-files:nil
End:
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