| From: | Robert Haas <robertmhaas(at)gmail(dot)com> |
|---|---|
| To: | Andres Freund <andres(at)anarazel(dot)de> |
| 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: | 2017-03-06 23:59:02 |
| Message-ID: | CA+TgmoZhOAAgFUQ-ke80yik6SAOs_C9=PPS1ndMUB-bUbEk5ZA@mail.gmail.com |
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| Lists: | pgsql-hackers |
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.
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
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