Re: count(*) using index scan in "query often, update rarely" environment - Mailing list pgsql-performance

From Cestmir Hybl
Subject Re: count(*) using index scan in "query often, update rarely" environment
Date
Msg-id 9fbe01c5cb30$4407d580$131fc39e@stratos
Whole thread Raw
In response to count(*) using index scan in "query often, update rarely" environment  ("Cestmir Hybl" <cestmirl@freeside.sk>)
Responses Re: count(*) using index scan in "query often, update rarely" environment  (Alvaro Herrera <alvherre@alvh.no-ip.org>)
List pgsql-performance
collision: it's possible to either block updating transaction until index
scan ends or discard index scan imediately and finish query using MVCC
compliant scan

dead rows: this sounds like more serious counter-argument, I don't know much
about dead records management and whether it would be  possible/worth to
make indexes matching live records when there's no transaction in progress
on that table

----- Original Message -----
From: "Steinar H. Gunderson" <sgunderson@bigfoot.com>
To: <pgsql-performance@postgresql.org>
Sent: Friday, October 07, 2005 12:48 PM
Subject: Re: [PERFORM] count(*) using index scan in "query often, update
rarely" environment


> On Fri, Oct 07, 2005 at 11:24:05AM +0200, Cestmir Hybl wrote:
>> Isn't it possible (and reasonable) for these environments to keep track
>> of
>> whether there is a transaction in progress with update to given table and
>> if not, use an index scan (count(*) where) or cached value (count(*)) to
>> perform this kind of query?
>
> Even if there is no running update, there might still be dead rows in the
> table. In any case, of course, a new update could always be occurring
> while
> your counting query was still running.
>
> /* Steinar */
> --
> Homepage: http://www.sesse.net/
>
> ---------------------------(end of broadcast)---------------------------
> TIP 2: Don't 'kill -9' the postmaster


pgsql-performance by date:

Previous
From: "Steinar H. Gunderson"
Date:
Subject: Re: count(*) using index scan in "query often, update rarely" environment
Next
From: "Merlin Moncure"
Date:
Subject: Re: Status of Opteron vs Xeon