Under what conditions does EXPLAIN ANALYZE report spilling work_mem to disk? When does it not report work_mem or other overflow to disk? I know that a planned disk-sort shows up. I have also seen it report a hash-agg on disk, but this was a while ago and rather difficult to reproduce and I'm somewhat confident I have seen it spill to temp disk without reporting it in EXPLAIN ANALYZE, but I could be wrong.
On Thu, Sep 18, 2008 at 11:13 AM, Tom Lane <[EMAIL PROTECTED]> wrote: > "Kevin Grittner" <[EMAIL PROTECTED]> writes: > > "Nikolas Everett" <[EMAIL PROTECTED]> 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 > > -- > Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) > To make changes to your subscription: > http://www.postgresql.org/mailpref/pgsql-performance >
