From: Tom Lane <tgl@sss.pgh.pa.us>
To: Andrew Gierth <andrew@tao11.riddles.org.uk>
Cc: Alexander Kuzmenkov <a.kuzmenkov@postgrespro.ru>
Cc: PostgreSQL-development <pgsql-hackers@postgresql.org>
Subject: Re: index-only count(*) for indexes supporting bitmap scans
Date: Wed, 12 Apr 2017 08:04:21 -0400
Message-ID: <8437.1491998661@sss.pgh.pa.us> (raw)
In-Reply-To: <87ziflvryn.fsf@news-spur.riddles.org.uk>
References: <239a8955-c0fc-f506-026d-c837e86c827b@postgrespro.ru>
<87ziflvryn.fsf@news-spur.riddles.org.uk>
List-Unsubscribe: <mailto:majordomo@postgresql.org?body=unsub%20pgsql-hackers>
Andrew Gierth <andrew@tao11.riddles.org.uk> writes:
> "Alexander" == Alexander Kuzmenkov <a.kuzmenkov@postgrespro.ru> writes:
> Alexander> Structurally, the patch consists of two major parts: a
> Alexander> specialized executor node
> Why?
> It strikes me that the significant fact here is not that we're doing
> count(*), but that we don't need any columns from the bitmap heap scan
> result. Rather than creating a whole new node, can't the existing
> bitmap heapscan be taught to skip fetching the actual table page in
> cases where it's all-visible, not lossy, and no columns are needed?
+1 ... while I hadn't actually looked at the code, it seemed to me that
anything like the optimization-as-described would be impossibly klugy
from the planner's standpoint. Your formulation sounds lots nicer.
Detecting that no columns are needed in the executor might be a bit tricky
because of the planner's habit of inserting a "physical tlist" to avoid a
projection step. (See also nearby discussion about custom scan planning.)
But we could fix that. I think one rule that would make sense is to
just disable the physical-tlist substitution if the relation's targetlist
is empty --- it wouldn't be buying much in such a case anyhow. Then the
runtime tlist for the scan node would also be empty, and away you go.
regards, tom lane
--
Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-hackers
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Reply to all the recipients using the --to and --cc options:
reply via email
To: pgsql-hackers@postgresql.org
Cc: tgl@sss.pgh.pa.us, andrew@tao11.riddles.org.uk, a.kuzmenkov@postgrespro.ru
Subject: Re: index-only count(*) for indexes supporting bitmap scans
In-Reply-To: <8437.1491998661@sss.pgh.pa.us>
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
This inbox is served by DDX for PostgreSQL; see mirroring instructions
for how to clone and mirror all data and code used for this inbox