"Kevin Grittner" <Kevin.Grittner@xxxxxxxxxxxx> writes: > "Nikolas Everett" <nik9000@xxxxxxxxx> 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
