diff options
| -rw-r--r-- | doc/src/sgml/ref/create_index.sgml | 16 |
1 files changed, 12 insertions, 4 deletions
diff --git a/doc/src/sgml/ref/create_index.sgml b/doc/src/sgml/ref/create_index.sgml index abd0987b15b..34e12c6a5d2 100644 --- a/doc/src/sgml/ref/create_index.sgml +++ b/doc/src/sgml/ref/create_index.sgml @@ -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 @@ -77,9 +77,17 @@ CREATE [ UNIQUE ] INDEX <replaceable class="parameter">name</replaceable> ON <re </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> |