Re: [HACKERS] Performance degradation in TPC-H Q18

On 2017-03-06 18:59:02 -0500, Robert Haas wrote:
> On Mon, Mar 6, 2017 at 5:19 PM, Andres Freund <andres@anarazel.de> wrote:
> > The whole performance issue trigger this thread only exists when the
> > hashtable sizes are mis-estimated, right?  Turns out that after applying
> > the just-committed changes, that "fixing" the bad-mixing and/or doing
> > iteration that's not entirely in hash-order, slighty degrades
> > performance.  The difference is that without either of those additional
> > changes, we resize to the "right" size very early, when the hashtable is
> > barely filled (i.e. only few entries need to be moved), because the
> > imbalance is observed at tsart.  With the changes however the resizing
> > happens when the table is pretty full (i.e. a lot of entries need to be
> > moved).  So the early imbalance ends up actually not hurting
> > performance...
> 
> Hmm.  I don't know what to do about that.

Oh, I don't think we need to do much about it either way - I think it's
more an interesting curiosity than anything.  Seems to reduce the
urgency for better iteration a bit, that's it.

- Andres