Re: Why does this query write to the disk? - Mailing list pgsql-performance

>>> "Nikolas Everett" <nik9000@gmail.com> wrote:

> I'm a bit confused as to why this query writes to the disk:
> SELECT count(*)
> FROM    bigbigtable
> WHERE customerid IN (SELECT customerid FROM
> smallcustomertable)
> AND x !=
> 'special'
>
> AND y IS NULL
>
> It writes a whole bunch of data to the disk that has the tablespace
where
> bigbigtable lives as well as writes a little data to the main disk.
It
> looks like its is actually WAL logging these writes.

It's probably writing hint bits to improve performance of subsequent
access to the table.  The issue is discussed here:

http://wiki.postgresql.org/wiki/Hint_Bits

-Kevin