| 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 05:18:57 |
| Message-ID: | 26447.1459660737@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 3:43 AM, Alex Shulgin <alex(dot)shulgin(at)gmail(dot)com> wrote:
>> I'm not sure yet about the 1% rule for the last value, but would also love
>> to see if we can avoid the arbitrary limit here. What happens with a last
>> value which is less than 1% popular in the current code anyway?
> Now that I think about it, I don't really believe this arbitrary heuristic
> is any good either, sorry.
Yeah, it was just a placeholder to produce a working patch.
Maybe we could base this cutoff on the stats target for the column?
That is, "1%" would be the right number if stats target is 100,
otherwise scale appropriately.
> What was your motivation to introduce some limit at the bottom anyway?
Well, we have to do *something* with the last (possibly only) value.
Neither "include always" nor "omit always" seem sane to me. What other
decision rule do you want there?
regards, tom lane
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