Add note that TRUNCATE uses an access exclusive lock. This apparently

surprised/confused some users.

doc/src/sgml/ref/truncate.sgml

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-$PostgreSQL: pgsql/doc/src/sgml/ref/truncate.sgml,v 1.30 2008/11/14 10:22:47 petere Exp $
+$PostgreSQL: pgsql/doc/src/sgml/ref/truncate.sgml,v 1.31 2008/12/18 10:45:00 petere Exp $
 PostgreSQL documentation
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@@ -102,6 +102,13 @@ TRUNCATE [ TABLE ] <replaceable class="PARAMETER">name</replaceable> [, ... ]
    to truncate it.
   </para>
 
+  <para>
+   <command>TRUNCATE</> acquires an access exclusive lock on the
+   tables in operates on, which blocks all other concurrent operations
+   on the table.  If concurrent access to a table is required, then
+   the <command>DELETE</> command should be used instead.
+  </para>
+
   <para>
    <command>TRUNCATE</> cannot be used on a table that has foreign-key
    references from other tables, unless all such tables are also truncated