- <sect3 id="querymodel-zebra-attr-approx">
- <title>Zebra Extension Approximative Limit (type 9)</title>
- </sect3>
- <para>
- The <literal>Zebra Extension Approximative Limit (type
- 9)</literal> is a way to enable approximate
- hit counts for <literal>scan</literal> hit counts, in the same
- way as for <literal>search</literal> hit counts.
- </para>
- <!--
+ <section id="querymodel-zebra-local-attr-limit">
+ <title>Local Approximative Limit Attribute (type 11)</title>
+ <para>
+ &zebra; computes - unless otherwise configured -
+ the exact hit count for every &acro.apt;
+ (leaf) in the query tree. These hit counts are returned as part of
+ the searchResult-1 facility in the binary encoded &acro.z3950; search
+ response packages.
+ </para>
+ <para>
+ By setting an estimation limit size of the resultset of the &acro.apt;
+ leaves, &zebra; stops processing the result set when the limit
+ length is reached.
+ Hit counts under this limit are still precise, but hit counts over it
+ are estimated using the statistics gathered from the chopped
+ result set.
+ </para>
+ <para>
+ Specifying a limit of <literal>0</literal> results in exact hit counts.
+ </para>
+ <para>
+ For example, we might be interested in exact hit count for a, but
+ for b we allow hit count estimates for 1000 and higher.
+ <screen>
+ Z> find @and a @attr 11=1000 b
+ </screen>
+ </para>
+ <note>
+ <para>
+ The estimated hit count facility makes searches faster, as one
+ only needs to process large hit lists partially.
+ It is mostly used in huge databases, where you you want trade
+ exactness of hit counts against speed of execution.
+ </para>
+ </note>
+ <warning>
+ <para>
+ Do not use approximative hit count limits
+ in conjunction with relevance ranking, as re-sorting of the
+ result set only works when the entire result set has
+ been processed.
+ </para>
+ </warning>
+ </section>
+
+ <section id="querymodel-zebra-global-attr-limit">
+ <title>Global Approximative Limit Attribute (type 12)</title>
+ <para>
+ By default &zebra; computes precise hit counts for a query as
+ a whole. Setting attribute 12 makes it perform approximative
+ hit counts instead. It has the same semantics as
+ <literal>estimatehits</literal> for the <xref linkend="zebra-cfg"/>.
+ </para>
+ <para>
+ The attribute (12) can occur anywhere in the query tree.
+ Unlike regular attributes it does not relate to the leaf (&acro.apt;)
+ - but to the whole query.
+ </para>
+ <warning>
+ <para>
+ Do not use approximative hit count limits
+ in conjunction with relevance ranking, as re-sorting of the
+ result set only works when the entire result set has
+ been processed.
+ </para>
+ </warning>
+ </section>
+
+ </section>
+
+ <section id="querymodel-zebra-attr-scan">
+ <title>&zebra; specific Scan Extensions to all Attribute Sets</title>