Re: FETCH FIRST clause WITH TIES option

From: Surafel Temesgen <surafel3000(at)gmail(dot)com>
To: Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>
Cc: 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: 2019-01-16 08:45:46
Message-ID: CALAY4q-+CKiFkgCL5VSK-9P8i5Z9uoQQh6ZEZLkjzsUT2mySsw@mail.gmail.com
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On Tue, Jan 15, 2019 at 2:51 PM Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>
wrote:

> What do you mean by "raw statistic"?
>

I mean without further calculation to consider other operation

>
> IMHO the best thing you can do is call estimate_num_groups() and combine
> that with the number of input rows. That shall benefit from ndistinct
> coefficients when available, etc. I've been thinking that considering
> the unreliability of grouping estimates we should use a multiple of the
> average size (because there may be much larger groups), but I think
> that's quite unprecipled and I'd much rather try without it first.
>
>
thank you for clarifying.

The attache patch use your suggestion uptread

regards

Surafel

Attachment Content-Type Size
fetch_first_with_ties_v5.patch text/x-patch 28.4 KB

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