| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com> |
| 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 19:08:05 |
| Message-ID: | 32413.1554318485@sss.pgh.pa.us |
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| Lists: | pgsql-hackers |
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.
regards, tom lane
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