index 992408ddc3539d1447a57dac353816425cff57d3..6d731e0701fdda4e2c89349d79a7816e26981b46 100644 (file)
@@ -90,6 +90,39 @@ CREATE INDEX test1_id_index ON test1 (id);
also significantly speed up queries with joins.
</para>
+ <para>
+ In general, <productname>PostgreSQL</productname> indexes can be used
+ to optimize queries that contain one or more <literal>WHERE</literal>
+ or <literal>JOIN</literal> clauses of the form
+
+<synopsis>
+<replaceable>indexed-column</replaceable> <replaceable>indexable-operator</replaceable> <replaceable>comparison-value</replaceable>
+</synopsis>
+
+ Here, the <replaceable>indexed-column</replaceable> is whatever
+ column or expression the index has been defined on.
+ The <replaceable>indexable-operator</replaceable> is an operator that
+ is a member of the index's <firstterm>operator class</firstterm> for
+ the indexed column. (More details about that appear below.)
+ And the <replaceable>comparison-value</replaceable> can be any
+ expression that is not volatile and does not reference the index's
+ table.
+ </para>
+
+ <para>
+ In some cases the query planner can extract an indexable clause of
+ this form from another SQL construct. A simple example is that if
+ the original clause was
+
+<synopsis>
+<replaceable>comparison-value</replaceable> <replaceable>operator</replaceable> <replaceable>indexed-column</replaceable>
+</synopsis>
+
+ then it can be flipped around into indexable form if the
+ original <replaceable>operator</replaceable> has a commutator
+ operator that is a member of the index's operator class.
+ </para>
+
<para>
Creating an index on a large table can take a long time. By default,
<productname>PostgreSQL</productname> allows reads (<command>SELECT</command> statements) to occur
@@ -120,7 +153,7 @@ CREATE INDEX test1_id_index ON test1 (id);
B-tree, Hash, GiST, SP-GiST, GIN, BRIN, and the extension <link
linkend="bloom">bloom</link>.
Each index type uses a different
- algorithm that is best suited to different types of queries.
+ algorithm that is best suited to different types of indexable clauses.
By default, the <link linkend="sql-createindex"><command>CREATE
INDEX</command></link> command creates
B-tree indexes, which fit the most common situations.