The following bug has been logged online: Bug reference: 4900 Logged by: Peter Headland Email address: pheadl...@actuate.com PostgreSQL version: 8.4.0 Operating system: Windows Description: Query planner misses obvious optimization on ordered UNION DISTINCT Details:
Consider the following union: SELECT a, b, c FROM t WHERE d = 1 UNION DISTINCT SELECT a, b, c FROM t WHERE d = 2 ORDER BY b, c; I have a table for which the plan for the above is ...->sort->unique->sort. I infer that the first sort is a,b,c. The obvious optimization is to reorder the columns used in the first sort to eliminate the need for the second sort. To illustrate this, I change the query to SELECT b, c, a FROM t WHERE d = 1 UNION DISTINCT SELECT b, c, a FROM t WHERE d = 2 ORDER BY b, c; the plan now becomes ...->sort->distinct. -- Sent via pgsql-bugs mailing list (pgsql-bugs@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-bugs
