Thanks. So am I right in assuming that the aggregate sub-query ( against work_active ) results will not assist with constraint exclusion in the sub-query against work_unit (if we introduce range partitions on this table)? Mr -----Original Message----- From: Tom Lane [mailto:tgl@xxxxxxxxxxxxx] Sent: Sunday, August 01, 2010 7:08 AM To: Mark Rostron Cc: pgsql-performance@xxxxxxxxxxxxxx Subject: Re: what does "initplan" operation in explain output mean? Mark Rostron <mrostron@xxxxxxx> writes: > This message is a request for information about the "initplan" operation in explain plan. An initplan is a sub-SELECT that only needs to be executed once because it has no dependency on the immediately surrounding query level. The cases you show here are from sub-SELECTs like this: (select min(wu_id) from work_active limit 1) which yields a value that's independent of anything in the outer query. If there were an outer reference in there, you'd get a SubPlan instead, because the subquery would need to be done over again for each row of the outer query. BTW, adding LIMIT 1 to an aggregate query is pretty pointless. regards, tom lane -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
