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Diffstat (limited to 'doc/src')
| -rw-r--r-- | doc/src/sgml/xindex.sgml | 18 |
1 files changed, 17 insertions, 1 deletions
diff --git a/doc/src/sgml/xindex.sgml b/doc/src/sgml/xindex.sgml index dcb5f9b6517..c6bfb0e19f3 100644 --- a/doc/src/sgml/xindex.sgml +++ b/doc/src/sgml/xindex.sgml @@ -1,5 +1,5 @@ <!-- -$Header: /cvsroot/pgsql/doc/src/sgml/xindex.sgml,v 1.24 2002/04/17 20:57:56 tgl Exp $ +$Header: /cvsroot/pgsql/doc/src/sgml/xindex.sgml,v 1.25 2002/05/29 17:36:40 tgl Exp $ PostgreSQL documentation --> @@ -105,6 +105,22 @@ PostgreSQL documentation </table> </para> + <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> + <para> The <acronym>OID</acronym> of the row in <filename>pg_am</filename> is used as a foreign key in a lot of other |