Re: Why does this query write to the disk?

"Kevin Grittner" <Kevin(dot)Grittner(at)wicourts(dot)gov> writes:
> "Nikolas Everett" <nik9000(at)gmail(dot)com> wrote:
>> I'm a bit confused as to why this query writes to the disk:

> 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

Hint-bit updates wouldn't be WAL-logged. If the table has been around a
long time, it might be freezing old tuples, which *would* be WAL-logged
(since 8.2 or so) --- but that would be a one-time, non-repeatable
behavior. How sure are you that there was WAL output?

What I was thinking was more likely was that the hash table for the hash
join was spilling out to temp files. That wouldn't be WAL-logged
either, but depending on your tablespace setup it might result in I/O on
some other disk than the table proper.

regards, tom lane