| From: | Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com> |
|---|---|
| To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
| Cc: | Surafel Temesgen <surafel3000(at)gmail(dot)com>, David Steele <david(at)pgmasters(dot)net>, Michael Paquier <michael(at)paquier(dot)xyz>, Robert Haas <robertmhaas(at)gmail(dot)com>, andrew(at)tao11(dot)riddles(dot)org(dot)uk, PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: FETCH FIRST clause WITH TIES option |
| Date: | 2019-04-03 19:40:59 |
| Message-ID: | 20190403194059.yqtuuwtsipq2vknh@development |
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| Lists: | pgsql-hackers |
On Wed, Apr 03, 2019 at 03:08:05PM -0400, Tom Lane wrote:
>Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com> writes:
>> I've tried to fix the merge conflict (essentially by moving some of the
>> code to adjust_limit_rows_costs(), but I'm wondering if the code added to
>> create_limit_path is actually correct
>> ...
>> Firstly, this seriously needs some comment explaining why we do this.
>
>I've not looked at this patch, but TBH I wonder why it is touching
>planner rowcount estimation at all. I find it doubtful either that
>a correction for WITH TIES would be significant in most use-cases,
>or that we could estimate it accurately if it was significant.
>It certainly doesn't seem like something that needs to be messed
>with in v1 of the feature.
>
FWIW it was me who suggested to tweak the cardinality estimation this way,
but if we want to leave it out from v1, I'm OK with that.
regards
--
Tomas Vondra http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
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