svn commit - mysqldoc@docsrva: r577 - trunk/refman-4.1

[stefan]

Author: shinz
Date: 2005-12-16 22:02:03 +0100 (Fri, 16 Dec 2005)
New Revision: 577

Log:
Loose index scans appear in 5.0 only (Domas)

Modified:
   trunk/refman-4.1/optimization.xml

Modified: trunk/refman-4.1/optimization.xml
===================================================================
--- trunk/refman-4.1/optimization.xml	2005-12-16 18:46:45 UTC (rev 576)
+++ trunk/refman-4.1/optimization.xml	2005-12-16 21:02:03 UTC (rev 577)
@@ -3409,137 +3409,6 @@
         performs a range scan, and then groups the resulting tuples.
       </para>
 
-      <section id="loose-index-scan">
-
-        <title>&title-loose-index-scan;</title>
-
-        <para>
-          The most efficient way is when the index is used to directly
-          retrieve the group fields. With this access method, MySQL uses
-          the property of some index types (for example, B-Trees) that
-          the keys are ordered. This property allows use of lookup
-          groups in an index without having to consider all keys in the
-          index that satisfy all <literal>WHERE</literal> conditions.
-          Since this access method considers only a fraction of the keys
-          in an index, it is called a <firstterm>loose index
-          scan</firstterm>. When there is no <literal>WHERE</literal>
-          clause, a loose index scan reads as many keys as the number of
-          groups, which may be a much smaller number than that of all
-          keys. If the <literal>WHERE</literal> clause contains range
-          predicates (see the discussion in <xref linkend="explain"/> of
-          the <literal>range</literal> join type), a loose index scan
-          looks up the first key of each group that satisfies the range
-          conditions, and again reads the least possible number of keys.
-          This is possible under the following conditions:
-        </para>
-
-        <itemizedlist>
-
-          <listitem>
-            <para>
-              The query is over a single table.
-            </para>
-          </listitem>
-
-          <listitem>
-            <para>
-              The <literal>GROUP BY</literal> includes the first
-              consecutive parts of the index (if instead of
-              <literal>GROUP BY</literal>, the query has a
-              <literal>DISTINCT</literal> clause, then all distinct
-              attributes refer to the beginning of the index).
-            </para>
-          </listitem>
-
-          <listitem>
-            <para>
-              The only aggregate functions used (if any) are
-              <literal>MIN()</literal> and <literal>MAX()</literal>, and
-              all of them refer to the same column.
-            </para>
-          </listitem>
-
-          <listitem>
-            <para>
-              Any other parts of the index than those from the
-              <literal>GROUP BY</literal> referenced in the query must
-              be constants (that is, they must be referenced in
-              equalities with constants), except for the argument of
-              <literal>MIN()</literal> or <literal>MAX()</literal>
-              functions.
-            </para>
-          </listitem>
-
-        </itemizedlist>
-
-        <para>
-          The <literal>EXPLAIN</literal> output for such queries shows
-          <literal>Using index for group-by</literal> in the
-          <literal>Extra</literal> column.
-        </para>
-
-        <para>
-          The following queries provide several examples that fall into
-          this category, assuming there is an index <literal>idx(c1, c2,
-          c3)</literal> on table <literal>t1(c1,c2,c3,c4)</literal>:
-        </para>
-
-<programlisting>
-SELECT c1, c2 FROM t1 GROUP BY c1, c2;
-SELECT DISTINCT c1, c2 FROM t1;
-SELECT c1, MIN(c2) FROM t1 GROUP BY c1;
-SELECT c1, c2 FROM t1 WHERE c1 &lt; <replaceable>const</replaceable> GROUP BY c1, c2;
-SELECT MAX(c3), MIN(c3), c1, c2 FROM t1 WHERE c2 &gt; <replaceable>const</replaceable> GROUP BY c1, c2;
-SELECT c2 FROM t1 WHERE c1 &lt; <replaceable>const</replaceable> GROUP BY c1, c2;
-SELECT c1, c2 FROM t1 WHERE c3 = <replaceable>const</replaceable> GROUP BY c1, c2;
-</programlisting>
-
-        <para>
-          The following queries cannot be executed with this quick
-          select method, for the reasons given:
-        </para>
-
-        <itemizedlist>
-
-          <listitem>
-            <para>
-              There are other aggregate functions than
-              <literal>MIN()</literal> or <literal>MAX()</literal>, for
-              example:
-            </para>
-
-<programlisting>
-SELECT c1, SUM(c2) FROM t1 GROUP BY c1;
-</programlisting>
-          </listitem>
-
-          <listitem>
-            <para>
-              The fields in the <literal>GROUP BY</literal> clause do
-              not refer to the beginning of the index, as shown here:
-            </para>
-
-<programlisting>
-SELECT c1,c2 FROM t1 GROUP BY c2, c3;
-</programlisting>
-          </listitem>
-
-          <listitem>
-            <para>
-              The query refers to a part of a key that comes after the
-              <literal>GROUP BY</literal> part, and for which there is
-              no equality with a constant, an example being:
-            </para>
-
-<programlisting>
-SELECT c1,c3 FROM t1 GROUP BY c1, c2;
-</programlisting>
-          </listitem>
-
-        </itemizedlist>
-
-      </section>
-
       <section id="tight-index-scan">
 
         <title>&title-tight-index-scan;</title>