$Header: /cvsroot/pgsql/doc/src/sgml/xindex.sgml,v 1.25 2002/05/29 17:36:40 tgl Exp $

<note>

<para>

An index AM that supports multiple columns (has

<structfield>amcanmulticol</structfield> true) <emphasis>must</>

support indexing nulls in columns after the first, because the planner

will assume the index can be used for queries on just the first

column(s). For example, consider an index on (a,b) and a query

WHERE a = 4. The system will assume the index can be used to scan for

rows with a = 4, which is wrong if the index omits rows where b is null.

However it is okay to omit rows where the first indexed column is null.

(GiST currently does so.)

<structfield>amindexnulls</structfield> should be set true only if the

index AM indexes all rows, including arbitrary combinations of nulls.

</para>

</note>