Doc: minor improvements for section 11.2 "Index Types".

Break the per-index-type discussions into <sect2>'s so as to make
them more visually separate and easier to find.  Improve the markup,
and make a couple of small wording adjustments.

This also fixes one stray reference to the now-deprecated point
operators <^ and >^.

Dagfinn Ilmari Mannsåker, reviewed by David Johnston and Jürgen Purtz

Discussion: https://postgr.es/m/877dukhvzg.fsf@wibble.ilmari.org

Author: tglsfdc

doc/src/sgml/indices.sgml

@@ -118,32 +118,39 @@ CREATE INDEX test1_id_index ON test1 (id);
    B-tree, Hash, GiST, SP-GiST, GIN and BRIN.
    Each index type uses a different
    algorithm that is best suited to different types of queries.
-   By default, the <command>CREATE INDEX</command> command creates
+   By default, the <link linkend="sql-createindex"><command>CREATE
+   INDEX</command></link> command creates
    B-tree indexes, which fit the most common situations.
+   The other index types are selected by writing the keyword
+   <literal>USING</literal> followed by the index type name.
+   For example, to create a Hash index:
+<programlisting>
+CREATE INDEX <replaceable>name</replaceable> ON <replaceable>table</replaceable> USING HASH (<replaceable>column</replaceable>);
+</programlisting>
   </para>
 
-  <para>
+  <sect2 id="indexes-types-btree">
+   <title>B-Tree</title>
+
    <indexterm>
     <primary>index</primary>
-    <secondary>B-tree</secondary>
+    <secondary>B-Tree</secondary>
    </indexterm>
    <indexterm>
-    <primary>B-tree</primary>
+    <primary>B-Tree</primary>
     <see>index</see>
    </indexterm>
+
+  <para>
    B-trees can handle equality and range queries on data that can be sorted
    into some ordering.
    In particular, the <productname>PostgreSQL</productname> query planner
    will consider using a B-tree index whenever an indexed column is
    involved in a comparison using one of these operators:
 
-   <simplelist>
-    <member><literal>&lt;</literal></member>
-    <member><literal>&lt;=</literal></member>
-    <member><literal>=</literal></member>
-    <member><literal>&gt;=</literal></member>
-    <member><literal>&gt;</literal></member>
-   </simplelist>
+<synopsis>
+&lt; &nbsp; &lt;= &nbsp; = &nbsp; &gt;= &nbsp; &gt;
+</synopsis>
 
    Constructs equivalent to combinations of these operators, such as
    <literal>BETWEEN</literal> and <literal>IN</literal>, can also be implemented with
@@ -172,8 +179,11 @@ CREATE INDEX test1_id_index ON test1 (id);
    This is not always faster than a simple scan and sort, but it is
    often helpful.
   </para>
+  </sect2>
+
+  <sect2 id="indexes-types-hash">
+   <title>Hash</title>
 
-  <para>
    <indexterm>
     <primary>index</primary>
     <secondary>hash</secondary>
@@ -182,17 +192,24 @@ CREATE INDEX test1_id_index ON test1 (id);
     <primary>hash</primary>
     <see>index</see>
    </indexterm>
-   Hash indexes can only handle simple equality comparisons.
+
+  <para>
+   Hash indexes store a 32-bit hash code derived from the
+   value of the indexed column. Hence,
+   such indexes can only handle simple equality comparisons.
    The query planner will consider using a hash index whenever an
    indexed column is involved in a comparison using the
-   <literal>=</literal> operator.
-   The following command is used to create a hash index:
+   equal operator:
+
 <synopsis>
-CREATE INDEX <replaceable>name</replaceable> ON <replaceable>table</replaceable> USING HASH (<replaceable>column</replaceable>);
+=
 </synopsis>
   </para>
+  </sect2>
+
+  <sect2 id="indexes-type-gist">
+   <title>GiST</title>
 
-  <para>
    <indexterm>
     <primary>index</primary>
     <secondary>GiST</secondary>
@@ -201,6 +218,8 @@ CREATE INDEX <replaceable>name</replaceable> ON <replaceable>table</replaceable>
     <primary>GiST</primary>
     <see>index</see>
    </indexterm>
+
+  <para>
    GiST indexes are not a single kind of index, but rather an infrastructure
    within which many different indexing strategies can be implemented.
    Accordingly, the particular operators with which a GiST index can be
@@ -210,20 +229,9 @@ CREATE INDEX <replaceable>name</replaceable> ON <replaceable>table</replaceable>
    for several two-dimensional geometric data types, which support indexed
    queries using these operators:
 
-   <simplelist>
-    <member><literal>&lt;&lt;</literal></member>
-    <member><literal>&amp;&lt;</literal></member>
-    <member><literal>&amp;&gt;</literal></member>
-    <member><literal>&gt;&gt;</literal></member>
-    <member><literal>&lt;&lt;|</literal></member>
-    <member><literal>&amp;&lt;|</literal></member>
-    <member><literal>|&amp;&gt;</literal></member>
-    <member><literal>|&gt;&gt;</literal></member>
-    <member><literal>@&gt;</literal></member>
-    <member><literal>&lt;@</literal></member>
-    <member><literal>~=</literal></member>
-    <member><literal>&amp;&amp;</literal></member>
-   </simplelist>
+<synopsis>
+&lt;&lt; &nbsp; &amp;&lt; &nbsp; &amp;&gt; &nbsp; &gt;&gt; &nbsp; &lt;&lt;| &nbsp; &amp;&lt;| &nbsp; |&amp;&gt; &nbsp; |&gt;&gt; &nbsp; @&gt; &nbsp; &lt;@ &nbsp; ~= &nbsp; &amp;&amp;
+</synopsis>
 
    (See <xref linkend="functions-geometry"/> for the meaning of
    these operators.)
@@ -246,8 +254,11 @@ SELECT * FROM places ORDER BY location <-> point '(101,456)' LIMIT 10;
    In <xref linkend="gist-builtin-opclasses-table"/>, operators that can be
    used in this way are listed in the column <quote>Ordering Operators</quote>.
   </para>
+  </sect2>
+
+  <sect2 id="indexes-type-spgist">
+   <title>SP-GiST</title>
 
-  <para>
    <indexterm>
     <primary>index</primary>
     <secondary>SP-GiST</secondary>
@@ -256,6 +267,8 @@ SELECT * FROM places ORDER BY location <-> point '(101,456)' LIMIT 10;
     <primary>SP-GiST</primary>
     <see>index</see>
    </indexterm>
+
+  <para>
    SP-GiST indexes, like GiST indexes, offer an infrastructure that supports
    various kinds of searches.  SP-GiST permits implementation of a wide range
    of different non-balanced disk-based data structures, such as quadtrees,
@@ -264,14 +277,9 @@ SELECT * FROM places ORDER BY location <-> point '(101,456)' LIMIT 10;
    for two-dimensional points, which support indexed
    queries using these operators:
 
-   <simplelist>
-    <member><literal>&lt;&lt;</literal></member>
-    <member><literal>&gt;&gt;</literal></member>
-    <member><literal>~=</literal></member>
-    <member><literal>&lt;@</literal></member>
-    <member><literal>&lt;^</literal></member>
-    <member><literal>&gt;^</literal></member>
-   </simplelist>
+<synopsis>
+&lt;&lt; &nbsp; &gt;&gt; &nbsp; ~= &nbsp; &lt;@ &nbsp; &lt;&lt;| &nbsp; |&gt;&gt;
+</synopsis>
 
    (See <xref linkend="functions-geometry"/> for the meaning of
    these operators.)
@@ -283,11 +291,14 @@ SELECT * FROM places ORDER BY location <-> point '(101,456)' LIMIT 10;
   <para>
    Like GiST, SP-GiST supports <quote>nearest-neighbor</quote> searches.
    For SP-GiST operator classes that support distance ordering, the
-   corresponding operator is specified in the <quote>Ordering Operators</quote>
+   corresponding operator is listed in the <quote>Ordering Operators</quote>
    column in <xref linkend="spgist-builtin-opclasses-table"/>.
   </para>
+  </sect2>
+
+  <sect2 id="indexes-types-gin">
+   <title>GIN</title>
 
-  <para>
    <indexterm>
     <primary>index</primary>
     <secondary>GIN</secondary>
@@ -296,6 +307,8 @@ SELECT * FROM places ORDER BY location <-> point '(101,456)' LIMIT 10;
     <primary>GIN</primary>
     <see>index</see>
    </indexterm>
+
+  <para>
    GIN indexes are <quote>inverted indexes</quote> which are appropriate for
    data values that contain multiple component values, such as arrays.  An
    inverted index contains a separate entry for each component value, and
@@ -312,12 +325,9 @@ SELECT * FROM places ORDER BY location <-> point '(101,456)' LIMIT 10;
    <productname>PostgreSQL</productname> includes a GIN operator class
    for arrays, which supports indexed queries using these operators:
 
-   <simplelist>
-    <member><literal>&lt;@</literal></member>
-    <member><literal>@&gt;</literal></member>
-    <member><literal>=</literal></member>
-    <member><literal>&amp;&amp;</literal></member>
-   </simplelist>
+<synopsis>
+&lt;@ &nbsp; @&gt; &nbsp; = &nbsp; &amp;&amp;
+</synopsis>
 
    (See <xref linkend="functions-array"/> for the meaning of
    these operators.)
@@ -327,8 +337,11 @@ SELECT * FROM places ORDER BY location <-> point '(101,456)' LIMIT 10;
    classes are available in the <literal>contrib</literal> collection or as separate
    projects.  For more information see <xref linkend="gin"/>.
   </para>
+  </sect2>
+
+  <sect2 id="indexes-types-brin">
+   <title>BRIN</title>
 
-  <para>
    <indexterm>
     <primary>index</primary>
     <secondary>BRIN</secondary>
@@ -337,8 +350,12 @@ SELECT * FROM places ORDER BY location <-> point '(101,456)' LIMIT 10;
     <primary>BRIN</primary>
     <see>index</see>
    </indexterm>
+
+  <para>
    BRIN indexes (a shorthand for Block Range INdexes) store summaries about
    the values stored in consecutive physical block ranges of a table.
+   Thus, they are most effective for columns whose values are well-correlated
+   with the physical order of the table rows.
    Like GiST, SP-GiST and GIN,
    BRIN can support many different indexing strategies,
    and the particular operators with which a BRIN index can be used
@@ -348,18 +365,15 @@ SELECT * FROM places ORDER BY location <-> point '(101,456)' LIMIT 10;
    values in the column for each block range.  This supports indexed queries
    using these operators:
 
-   <simplelist>
-    <member><literal>&lt;</literal></member>
-    <member><literal>&lt;=</literal></member>
-    <member><literal>=</literal></member>
-    <member><literal>&gt;=</literal></member>
-    <member><literal>&gt;</literal></member>
-   </simplelist>
+<synopsis>
+&lt; &nbsp; &lt;= &nbsp; = &nbsp; &gt;= &nbsp; &gt;
+</synopsis>
 
    The BRIN operator classes included in the standard distribution are
    documented in <xref linkend="brin-builtin-opclasses-table"/>.
    For more information see <xref linkend="brin"/>.
   </para>
+  </sect2>
  </sect1>