| From: | Peter Geoghegan <pg(at)bowt(dot)ie> |
|---|---|
| To: | Thomas Munro <thomas(dot)munro(at)enterprisedb(dot)com> |
| Cc: | Bhushan Uparkar <bhushan(dot)uparkar(at)gmail(dot)com>, Jesper Pedersen <jesper(dot)pedersen(at)redhat(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>, Andrew Dunstan <andrew(dot)dunstan(at)2ndquadrant(dot)com>, Alexander Korotkov <a(dot)korotkov(at)postgrespro(dot)ru> |
| Subject: | Re: Index Skip Scan |
| Date: | 2018-08-17 17:52:05 |
| Message-ID: | CAH2-Wzkk0+vSAiaRSkX84KR0SofKkY1yGHEcLoRBXT-JjM37bw@mail.gmail.com |
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| Lists: | pgsql-hackers |
On Thu, Aug 16, 2018 at 4:10 PM, Thomas Munro
<thomas(dot)munro(at)enterprisedb(dot)com> wrote:
> Can you give an example of problematic ndistinct underestimation?
Yes. See https://postgr.es/m/CAKuK5J12QokFh88tQz-oJMSiBg2QyjM7K7HLnbYi3Ze+Y5BtWQ@mail.gmail.com,
for example. That's a complaint about an underestimation specifically.
This seems to come up about once every 3 years, at least from my
perspective. I'm always surprised that ndistinct doesn't get
implicated in bad query plans more frequently.
> I suppose you might be able to defend against that in the executor: if
> you find that you've done an unexpectedly high number of skips, you
> could fall back to regular next-tuple mode. Unfortunately that's
> require the parent plan node to tolerate non-unique results.
I like the idea of dynamic fallback in certain situations, but the
details always seem complicated.
--
Peter Geoghegan
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