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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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60
doc/src/sgml/advanced.sgml
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@ -1,5 +1,5 @@
<!--
$PostgreSQL: pgsql/doc/src/sgml/advanced.sgml,v 1.41 2004/03/31 16:20:53 momjian Exp $
$PostgreSQL: pgsql/doc/src/sgml/advanced.sgml,v 1.42 2004/08/01 17:32:11 tgl Exp $
-->
<chapter id="tutorial-advanced">
@ -257,6 +257,64 @@ COMMIT;
you are using.
</para>
</note>
<para>
It's possible to control the statements in a transaction in a more
granular fashion through the use of <firstterm>savepoints</>. Savepoints
allow you to selectively discard parts of the transaction, while
committing the rest. After defining a savepoint with
<command>SAVEPOINT</>, you can if needed roll back to the savepoint
with <command>ROLLBACK TO</>. All the transaction's database changes
between defining the savepoint and rolling back to it are discarded, but
changes earlier than the savepoint are kept.
</para>
<para>
After rolling back to a savepoint, it continues to be defined, so you can
roll back to it several times. Conversely, if you are sure you won't need
to roll back to a particular savepoint again, it can be released, so the
system can free some resources. Keep in mind that either releasing or
rolling back to a savepoint
will automatically release all savepoints that were defined after it.
</para>
<para>
All this is happening within the transaction block, so none of it
is visible to other database sessions. When and if you commit the
transaction block, the committed actions become visible as a unit
to other sessions, while the rolled-back actions never become visible
at all.
</para>
<para>
Remembering the bank database, suppose we debit $100.00 from Alice's
account, and credit Bob's account, only to find later that we should
have credited Wally's account. We could do it using savepoints like
<programlisting>
BEGIN;
UPDATE accounts SET balance = balance - 100.00
WHERE name = 'Alice';
SAVEPOINT my_savepoint;
UPDATE accounts SET balance = balance + 100.00
WHERE name = 'Bob';
-- oops ... forget that and use Wally's account
ROLLBACK TO my_savepoint;
UPDATE accounts SET balance = balance + 100.00
WHERE name = 'Wally';
COMMIT;
</programlisting>
</para>
<para>
This example is, of course, oversimplified, but there's a lot of control
to be had over a transaction block through the use of savepoints.
Moreover, <command>ROLLBACK TO</> is the only way to regain control of a
transaction block that was put in aborted state by the
system due to an error, short of rolling it back completely and starting
again.
</para>
</sect1>

5
doc/src/sgml/ref/allfiles.sgml
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@ -1,5 +1,5 @@
<!--
$PostgreSQL: pgsql/doc/src/sgml/ref/allfiles.sgml,v 1.59 2004/06/25 21:55:50 tgl Exp $
$PostgreSQL: pgsql/doc/src/sgml/ref/allfiles.sgml,v 1.60 2004/08/01 17:32:13 tgl Exp $
PostgreSQL documentation
Complete list of usable sgml source files in this directory.
-->
@ -88,9 +88,12 @@ Complete list of usable sgml source files in this directory.
<!entity notify system "notify.sgml">
<!entity prepare system "prepare.sgml">
<!entity reindex system "reindex.sgml">
<!entity releaseSavepoint system "release.sgml">
<!entity reset system "reset.sgml">
<!entity revoke system "revoke.sgml">
<!entity rollback system "rollback.sgml">
<!entity rollbackTo system "rollback_to.sgml">
<!entity savepoint system "savepoint.sgml">
<!entity select system "select.sgml">
<!entity selectInto system "select_into.sgml">
<!entity set system "set.sgml">

5
doc/src/sgml/ref/begin.sgml
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@ -1,5 +1,5 @@
<!--
$PostgreSQL: pgsql/doc/src/sgml/ref/begin.sgml,v 1.30 2004/01/11 09:24:17 petere Exp $
$PostgreSQL: pgsql/doc/src/sgml/ref/begin.sgml,v 1.31 2004/08/01 17:32:13 tgl Exp $
PostgreSQL documentation
-->
@ -31,7 +31,7 @@ BEGIN [ WORK | TRANSACTION ]
<para>
<command>BEGIN</command> initiates a transaction block, that is,
all statements after <command>BEGIN</command> command will be
all statements after a <command>BEGIN</command> command will be
executed in a single transaction until an explicit <xref
linkend="sql-commit" endterm="sql-commit-title"> or <xref
linkend="sql-rollback" endterm="sql-rollback-title"> is given.
@ -145,6 +145,7 @@ BEGIN;
<member><xref linkend="sql-commit" endterm="sql-commit-title"></member>
<member><xref linkend="sql-rollback" endterm="sql-rollback-title"></member>
<member><xref linkend="sql-start-transaction" endterm="sql-start-transaction-title"></member>
<member><xref linkend="sql-savepoint" endterm="sql-savepoint-title"></member>
</simplelist>
</refsect1>
</refentry>

143
doc/src/sgml/ref/release.sgml Normal file
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@ -0,0 +1,143 @@
<!--
$PostgreSQL: pgsql/doc/src/sgml/ref/release.sgml,v 1.1 2004/08/01 17:32:13 tgl Exp $
PostgreSQL documentation
-->
<refentry id="SQL-RELEASE">
<refmeta>
<refentrytitle id="SQL-RELEASE-TITLE">RELEASE</refentrytitle>
<refmiscinfo>SQL - Language Statements</refmiscinfo>
</refmeta>
<refnamediv>
<refname>RELEASE</refname>
<refpurpose>destroy a previously defined savepoint</refpurpose>
</refnamediv>
<indexterm zone="sql-release">
<primary>RELEASE</primary>
</indexterm>
<indexterm zone="sql-release">
<primary>savepoints</primary>
<secondary>releasing</secondary>
</indexterm>
<refsynopsisdiv>
<synopsis>
RELEASE <replaceable>savepoint_name</replaceable>
</synopsis>
</refsynopsisdiv>
<refsect1>
<title>Description</title>
<para>
<command>RELEASE</command> destroys a savepoint previously defined
in the current transaction.
</para>
<para>
Destroying a savepoint makes it unavailable as a rollback point,
but it has no other user visible behavior. It does not undo the
effects of commands executed after the savepoint was established.
(To do that, see <xref linkend="sql-rollback-to"
endterm="sql-rollback-to-title">.) Destroying a savepoint when
it is no longer needed may allow the system to reclaim some resources
earlier than transaction end.
</para>
<para>
<command>RELEASE</command> also destroys all savepoints that were
established after the named savepoint was established.
</para>
</refsect1>
<refsect1>
<title>Parameters</title>
<variablelist>
<varlistentry>
<term><replaceable>savepoint_name</replaceable></term>
<listitem>
<para>
The name of the savepoint to destroy.
</para>
</listitem>
</varlistentry>
</variablelist>
</refsect1>
<refsect1>
<title>Notes</title>
<para>
Specifying a savepoint name that was not previously defined is an error.
</para>
<para>
It is not possible to release a savepoint when the transaction is in
aborted state.
</para>
<para>
If multiple savepoints have the same name, only the one that was most
recently defined is released.
</para>
</refsect1>
<refsect1>
<title>Examples</title>
<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>
RELEASE is fully conforming to the SQL standard.
</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-rollback" endterm="sql-rollback-title"></member>
<member><xref linkend="sql-rollback-to" endterm="sql-rollback-to-title"></member>
<member><xref linkend="sql-savepoint" endterm="sql-savepoint-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
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:
-->

3
doc/src/sgml/ref/rollback.sgml
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@ -1,5 +1,5 @@
<!--
$PostgreSQL: pgsql/doc/src/sgml/ref/rollback.sgml,v 1.17 2003/11/29 19:51:39 pgsql Exp $
$PostgreSQL: pgsql/doc/src/sgml/ref/rollback.sgml,v 1.18 2004/08/01 17:32:13 tgl Exp $
PostgreSQL documentation
-->
@ -90,6 +90,7 @@ ROLLBACK;
<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-rollback-to" endterm="sql-rollback-to-title"></member>
</simplelist>
</refsect1>
</refentry>

163
doc/src/sgml/ref/rollback_to.sgml Normal file
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@ -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
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:
-->

152
doc/src/sgml/ref/savepoint.sgml Normal file
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@ -0,0 +1,152 @@
<!--
$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
sgml-omittag:nil
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:
-->

3
doc/src/sgml/ref/start_transaction.sgml
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@ -1,5 +1,5 @@
<!--
$PostgreSQL: pgsql/doc/src/sgml/ref/start_transaction.sgml,v 1.11 2004/01/11 05:46:58 neilc Exp $
$PostgreSQL: pgsql/doc/src/sgml/ref/start_transaction.sgml,v 1.12 2004/08/01 17:32:13 tgl Exp $
PostgreSQL documentation
-->
@ -66,6 +66,7 @@ START TRANSACTION
<member><xref linkend="sql-commit" endterm="sql-commit-title"></member>
<member><xref linkend="sql-rollback" endterm="sql-rollback-title"></member>
<member><xref linkend="sql-set-transaction" endterm="sql-set-transaction-title"></member>
<member><xref linkend="sql-savepoint" endterm="sql-savepoint-title"></member>
</simplelist>
</refsect1>
</refentry>

5
doc/src/sgml/reference.sgml
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@ -1,5 +1,5 @@
<!-- reference.sgml
$PostgreSQL: pgsql/doc/src/sgml/reference.sgml,v 1.50 2004/06/25 21:55:51 tgl Exp $
$PostgreSQL: pgsql/doc/src/sgml/reference.sgml,v 1.51 2004/08/01 17:32:11 tgl Exp $
PostgreSQL Reference Manual
-->
@ -120,9 +120,12 @@ PostgreSQL Reference Manual
&notify;
&prepare;
&reindex;
&releaseSavepoint;
&reset;
&revoke;
&rollback;
&rollbackTo;
&savepoint;
&select;
&selectInto;
&set;