Re: Why does this query write to the disk?

"Kevin Grittner"  writes:
> "Nikolas Everett"  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