Hi,
indices.sgml contains this paragraph about hash indexes:
Note: Testing has shown PostgreSQL's hash indexes to perform no
better than B-tree indexes, and the index size and build time for hash
indexes is much worse. Furthermore, hash index operations are not
presently WAL-logged, so hash indexes might need to be rebuilt with
REINDEX after a database crash. For these reasons, hash index use is
presently discouraged.
However, it seems to me that hash indexes are much improved in 8.4, so
maybe this needs to be reworded. I'm not sure to what point they have
been improved though.
--
Alvaro Herrera http://www.CommandPrompt.com/
PostgreSQL Replication, Consulting, Custom Development, 24x7 support