From: | Andres Freund <andres(at)anarazel(dot)de> |
---|---|
To: | Robert Haas <robertmhaas(at)gmail(dot)com> |
Cc: | Kuntal Ghosh <kuntalghosh(dot)2007(at)gmail(dot)com>, Dilip Kumar <dilipbalaut(at)gmail(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: Performance degradation in TPC-H Q18 |
Date: | 2017-03-07 00:04:19 |
Message-ID: | 20170307000419.mra4gerd245q4jxz@alap3.anarazel.de |
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Lists: | pgsql-hackers |
On 2017-03-06 18:59:02 -0500, Robert Haas wrote:
> On Mon, Mar 6, 2017 at 5:19 PM, Andres Freund <andres(at)anarazel(dot)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
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