</listitem>

<listitem>

<para>

Old versions of updated rows can be completely removed during normal

operation, including <command>SELECT</command>s, instead of requiring

periodic vacuum operations. (This is possible because indexes

do not reference their <link linkend="storage-page-layout">page

item identifiers</link>.)

When a row is updated multiple times, row versions other than the oldest

and the newest can be completely removed during normal operation,

including <command>SELECT</command>s, instead of requiring periodic vacuum

operations. (Indexes always refer to the

<link linkend="storage-page-layout">page item identifier</link> of the

original row version. The tuple data associated with that row version

is removed, and its item identifier is converted to a redirect that

points to the oldest version that may still be visible to some concurrent

transaction. Intermediate row versions that are no longer visible to

anyone are completely removed, and the associated page item identifiers

are made available for reuse.)

</para>

</listitem>

</itemizedlist>