Re: I/O on select count(*)

>>> Doug Eck <deck1(at)yahoo(dot)com> wrote:

> I am attempting to run a query to determine the number of rows for a
given
> day using something like "select count(*) from tbl1 where ts between

> '2008-05-12 00:00:00.000' and '2008-05-12 23:59:59.999'". Explain
tells me
> that the query will be done using an index scan (as I would expect),
and I
> realize that it is going to take a while. My question concerns some
unusual
> I/O activity on the box (SUSE) when I run the query.
>
> For the first couple of minutes I see reads only. After that vmstat
shows
> mixed reads and writes in a ratio of about 1 block read to 5 blocks
written.

> Any thoughts into what could be going on? Thanks in advance for your
help.

Odd as it may seem, a SELECT can cause a page to be rewritten.

If this is the first time that the rows are being read since they were
inserted (or since the database was loaded, including from backup), it
may be rewriting the rows to set hint bits, which can make subsequent
access faster.

The best solution may be to vacuum more often.

http://archives.postgresql.org/pgsql-performance/2007-12/msg00206.php

-Kevin