Re: FETCH FIRST clause PERCENT option

From: Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>
To: Surafel Temesgen <surafel3000(at)gmail(dot)com>
Cc: vik(dot)fearing(at)2ndquadrant(dot)com, Mark Dilger <hornschnorter(at)gmail(dot)com>, Andres Freund <andres(at)anarazel(dot)de>, PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org>, andrew(at)tao11(dot)riddles(dot)org(dot)uk
Subject: Re: FETCH FIRST clause PERCENT option
Date: 2019-01-09 17:18:06
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On 1/9/19 4:43 PM, Surafel Temesgen wrote:
> On Wed, Jan 9, 2019 at 5:38 PM Tomas Vondra
> <tomas(dot)vondra(at)2ndquadrant(dot)com <mailto:tomas(dot)vondra(at)2ndquadrant(dot)com>> wrote:
> It's hard to say what exactly are you doing, because you haven't shared
> any code nor examples.
> okay i attach in progress patch

Yeah, that's what I thought - the patch computes

node->count = DatumGetInt64(100 / DatumGetFloat8(val));

and then always fetches this number of records before emitting the next
row from the tuplestore. That's wrong, as I explained before, because
the distance does change, due to rounding.

See the attached patch, which recomputes the count regularly. I don't
claim the patch is committable or that it has no other bugs, but
hopefully it shows what I meant.

FWIW you don't need to create any slots - the two already created are
enough. You certainly don't need to create the slots within a loop.


Tomas Vondra
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

Attachment Content-Type Size
percent-incremental.patch text/x-patch 27.8 KB

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