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Add note that TRUNCATE is not MVCC-safe.

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Tom Lane 2007-04-07 17:12:15 +00:00
parent b0194ab110
commit d7e2de6629
1 changed files with 25 additions and 2 deletions
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27
doc/src/sgml/ref/truncate.sgml
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@ -1,5 +1,5 @@
<!--
$PostgreSQL: pgsql/doc/src/sgml/ref/truncate.sgml,v 1.22 2007/01/31 23:26:04 momjian Exp $
$PostgreSQL: pgsql/doc/src/sgml/ref/truncate.sgml,v 1.23 2007/04/07 17:12:15 tgl Exp $
PostgreSQL documentation
-->
@ -31,7 +31,9 @@ TRUNCATE [ TABLE ] <replaceable class="PARAMETER">name</replaceable> [, ...] [ C
<command>TRUNCATE</command> quickly removes all rows from a set of
tables. It has the same effect as an unqualified
<command>DELETE</command> on each table, but since it does not actually
scan the tables it is faster. This is most useful on large tables.
scan the tables it is faster; furthermore it reclaims disk space
immediately, rather than requiring a subsequent vacuum operation.
This is most useful on large tables.
</para>
</refsect1>
@ -92,6 +94,27 @@ TRUNCATE [ TABLE ] <replaceable class="PARAMETER">name</replaceable> [, ...] [ C
<command>TRUNCATE</> will not run any user-defined <literal>ON
DELETE</literal> triggers that might exist for the tables.
</para>
<para>
<command>TRUNCATE</> is not MVCC-safe (see <xref linkend="mvcc">
for general information about MVCC). After truncation, the table
will appear empty to all concurrent transactions, even if they are
using a snapshot taken before the truncation occurred. This will
only be an issue for a transaction that did not touch the table
before the truncation started &mdash; any transaction that has done
so would hold at least <literal>ACCESS SHARE</literal> lock,
which would block
<command>TRUNCATE</> until that transaction completes. So
truncation will not cause any apparent inconsistency in the table
contents for successive queries on the same table, but it could
cause visible inconsistency between the contents of the
truncated table and other tables.
</para>
<para>
<command>TRUNCATE</> is transaction-safe, however: the truncation
will roll back if the surrounding transaction does not commit.
</para>
</refsect1>
<refsect1>