2012/8/26 Tom Lane <tgl@xxxxxxxxxxxxx>: > Jeff Janes <jeff.janes@xxxxxxxxx> writes: >> On Sun, Aug 26, 2012 at 8:02 AM, Thomas Kellerer <spam_eater@xxxxxxx> wrote: >>> Should the following setup qualify for an index scan? > >> ... Also, your filler is highly compressible, which means the table is >> much smaller than you might think. > > Yeah. I see something like 100 rows per page with this example; the > heap is 935 pages, the index 276, which makes things about a wash I/O > wise when you assume that random reads from the index will cost 4x what > sequential reads from the heap will. > > You can force an index scan to occur anyway by setting enable_seqscan to > zero. When I do that, I see an estimated cost that is marginally more > than for the seqscan, and the actual runtime is too. I'm not sure I'd > put a whole lot of stock in that considering the example is small enough > to be fully cached, but it does show that index-only scans aren't a > magic bullet. is possible use seqscan for index? When index is small - and can be smaller than related table. Regards Pavel > > regards, tom lane > > > -- > Sent via pgsql-general mailing list (pgsql-general@xxxxxxxxxxxxxx) > To make changes to your subscription: > http://www.postgresql.org/mailpref/pgsql-general -- Sent via pgsql-general mailing list (pgsql-general@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
