On Wed, Feb 22, 2012 at 10:02 PM, Robert Haas <robertmh...@gmail.com> wrote: > On Wed, Feb 22, 2012 at 2:23 PM, Simon Riggs <si...@2ndquadrant.com> wrote: >> The industry accepted description for non-sequential access is "random >> access" whether or not the function that describes the movement is >> entirely random. To argue otherwise is merely hairsplitting. > > I don't think so.
PostgreSQL already uses a parameter called "random_page_cost" to describe non-sequential access. Perhaps that is wrong and we need a third parameter? > For example, a bitmap index scan contrives to speed > things up by arranging for the table I/O to happen in ascending block > number order, with skips, rather than in random order, as a plain > index scan would do, and that seems to be a pretty effective > technique. Except to the extent that it interferes with the kernel's > ability to do readahead, it really can't be to read blocks 1, 2, 3, 4, > and 5 than to read blocks 1, 2, 4, and 5. Not reading block 3 can't > require more effort than reading it. By that argument, ANALYZE never could run longer than VACUUM ANALYZE, so you disagree with Tom and I and you can't explain Pavel's results.... cost_bitmap_heap_scan() uses "random_page_cost" to evaluate the cost of accessing blocks, even though the author knew the access was in ascending block number order. Why was that? Note that the cost_bitmap_heap_scan() cost can be > than cost-seqscan() for certain parameter values. -- Simon Riggs http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training & Services -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
