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> >> My question is whether postgres can index null values, and if not, do I

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> >> have to accept a full table scan when locating records.
> >
> > It indexes them, but "is null" is not an indexable operator, so you
> > can't directly solve the above with a 3-column index.  What you can do
> > instead is use a partial index, for instance
> >
> > create index i on CUSTOMER.WCCustOrderStatusLog (WCOrderStatusID)
> > where Acknowledged is null and Processing is null;
>
> That's a very nifty trick and exactly the sort of answer I was after!

Add CREATE INDEX doc mention of using partial indexes for IS NULL
indexing;  idea from Tom.
This commit is contained in:
Bruce Momjian 2004-04-20 01:00:26 +00:00
parent 6949fc0269
commit 6ef77149f7
1 changed files with 12 additions and 4 deletions
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16
doc/src/sgml/ref/create_index.sgml
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@ -1,5 +1,5 @@
<!--
$PostgreSQL: pgsql/doc/src/sgml/ref/create_index.sgml,v 1.44 2003/11/29 19:51:38 pgsql Exp $
$PostgreSQL: pgsql/doc/src/sgml/ref/create_index.sgml,v 1.45 2004/04/20 01:00:26 momjian Exp $
PostgreSQL documentation
-->
@ -66,7 +66,7 @@ CREATE [ UNIQUE ] INDEX <replaceable class="parameter">name</replaceable> ON <re
When the <literal>WHERE</literal> clause is present, a
<firstterm>partial index</firstterm> is created.
A partial index is an index that contains entries for only a portion of
a table, usually a portion that is somehow more interesting than the
a table, usually a portion that is more useful for indexing than the
rest of the table. For example, if you have a table that contains both
billed and unbilled orders where the unbilled orders take up a small
fraction of the total table and yet that is an often used section, you
@ -76,10 +76,18 @@ CREATE [ UNIQUE ] INDEX <replaceable class="parameter">name</replaceable> ON <re
table.
</para>
<para>
<literal>NULL</> values are not indexed by default. The best way
to index <literal>NULL</> values is to create a partial index using
an <literal>IS NULL</> comparison. <literal>IS NULL</> is more
of a function call than a value comparison, and this is why a partial
index works.
</para>
<para>
The expression used in the <literal>WHERE</literal> clause may refer
only to columns of the underlying table (but it can use all columns,
not only the one(s) being indexed). Presently, subqueries and
only to columns of the underlying table, but it can use all columns,
not just the ones being indexed. Presently, subqueries and
aggregate expressions are also forbidden in <literal>WHERE</literal>.
The same restrictions apply to index fields that are expressions.
</para>