| From: | David Rowley <dgrowleyml(at)gmail(dot)com> |
|---|---|
| To: | Andrei Lepikhov <lepihov(at)gmail(dot)com> |
| Cc: | pgsql-hackers(at)postgresql(dot)org, Tomas Vondra <tomas(dot)vondra(at)enterprisedb(dot)com> |
| Subject: | Re: Incremental Sort Cost Estimation Instability |
| Date: | 2024-09-12 10:12:03 |
| Message-ID: | CAApHDvpKFyRJTZco4=Y9_seZC5J02+G-S9UHFoUz+Zz1oF_w7A@mail.gmail.com |
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| Lists: | pgsql-hackers |
On Thu, 12 Sept 2024 at 21:51, Andrei Lepikhov <lepihov(at)gmail(dot)com> wrote:
> Initial problem causes wrong cost_sort estimation. Right now I think
> about providing cost_sort() the sort clauses instead of (or in addition
> to) the pathkeys.
I'm not quite sure why the sort clauses matter any more than the
EquivalenceClass. If the EquivalanceClass defines that all members
will have the same value for any given row, then, if we had to choose
any single member to drive the n_distinct estimate from, isn't the
most accurate distinct estimate from the member with the smallest
n_distinct estimate? (That assumes the less distinct member has every
value the more distinct member has, which might not be true)
David
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