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Doc: add a bit to indices.sgml about what is an indexable clause.

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We didn't explain this clearly until somewhere deep in the
"Extending SQL" chapter, but really it ought to be mentioned
in the introductory material too.

Discussion: https://postgr.es/m/4097442.1694967650@sss.pgh.pa.us
This commit is contained in:
Tom Lane 2023-12-17 16:49:44 -05:00
parent 3c9d9acae0
commit b7412e293b
1 changed files with 34 additions and 1 deletions
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35
doc/src/sgml/indices.sgml
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@ -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.