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

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:

Reply via email to