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-$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
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<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>
<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 — 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>