If we access a 1 block table using a SeqScan then it costs seq_page_cost, or 1 by default.
If we access the same 1 block table using an IndexScan then the access costs random_page_cost to access the index block and then random_page_cost to access to the data block. So the same block accessed for different reasons is judged to have two different costs. But that clearly must be wrong, since the I/O is a 1 block I/O in either case, This leads to favouring seq scans in cases where the actual index scan timing is actually less than seq scan. Proposal: Make the first block of a seq scan cost random_page_cost, then after that every additional block costs seq_page_cost. -- Simon Riggs www.2ndQuadrant.com PostgreSQL Training, Services and Support -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
