| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Alex Shulgin <alex(dot)shulgin(at)gmail(dot)com> |
| Cc: | "Shulgin, Oleksandr" <oleksandr(dot)shulgin(at)zalando(dot)de>, Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>, PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org>, David Steele <david(at)pgmasters(dot)net> |
| Subject: | Re: More stable query plans via more predictable column statistics |
| Date: | 2016-04-03 16:40:03 |
| Message-ID: | 18797.1459701603@sss.pgh.pa.us |
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| Lists: | pgsql-hackers |
Alex Shulgin <alex(dot)shulgin(at)gmail(dot)com> writes:
> On Sun, Apr 3, 2016 at 7:49 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> If there is only one value, it will have 100% of the samples, so it would
>> get included under just about any decision rule (other than "more than
>> 100% of this value plus following values"). I don't think making sure
>> this case works is sufficient to get us to a reasonable rule --- it's
>> a necessary case, but not a sufficient case.
> Well, if it's the only value it will be accepted simply because we are
> checking that special case already and don't even bother to loop through
> the track list.
That was demonstrably not the case in the failing regression test.
I forget what aspect of the test case allowed it to get past the short
circuit, but it definitely got into the scan-the-track-list code.
regards, tom lane
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