The following bug has been logged online: Bug reference: 5231 Logged by: Thomas Hamilton Email address: thomashamilto...@yahoo.com PostgreSQL version: 8.3.8 Operating system: Ubuntu 4.2.4 Description: SELECT DISTINCT poorly implemented vs SELECT ... GROUP BY Details:
SELECT DISTINCT does a Sort followed by Unique. SELECT ... GROUP BY, which is logically equivalent, performs a HashAggregate. When run against a large dataset with a small number of distinct results HashAggregate is an order of magnitude more efficient! Since the spec does not require DISTINCT to return sorted results, I don't believe Sort ... Unique will ever be more efficient than HashAggregate. Therefore, in order to maximize performance, DISTINCT should always be implemented as HashAggregate. -- Sent via pgsql-bugs mailing list (pgsql-bugs@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-bugs
