From: | Robert Haas <robertmhaas(at)gmail(dot)com> |
---|---|
To: | david(at)lang(dot)hm |
Cc: | Vitalii Tymchyshyn <tivv00(at)gmail(dot)com>, Jon Nelson <jnelson+pgsql(at)jamponi(dot)net>, Mladen Gogala <mladen(dot)gogala(at)vmsinfo(dot)com>, Andrew Dunstan <andrew(at)dunslane(dot)net>, Bruce Momjian <bruce(at)momjian(dot)us>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Craig Ringer <craig(at)postnewspapers(dot)com(dot)au>, "pgsql-performance(at)postgresql(dot)org" <pgsql-performance(at)postgresql(dot)org> |
Subject: | Re: [HACKERS] Slow count(*) again... |
Date: | 2011-02-04 01:29:14 |
Message-ID: | AANLkTi=Kx6rRFs3Kz2m32nLKwLGv0Ee87UF8ngKhppgM@mail.gmail.com |
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Lists: | pgsql-hackers pgsql-performance |
On Thu, Feb 3, 2011 at 7:39 PM, <david(at)lang(dot)hm> wrote:
>> Yeah, but you'll be passing the entire table through this separate
>> process that may only need to see 1% of it or less on a large table.
>> If you want to write the code and prove it's better than what we have
>> now, or some other approach that someone else may implement in the
>> meantime, hey, this is an open source project, and I like improvements
>> as much as the next guy. But my prediction for what it's worth is
>> that the results will suck. :-)
>
> I will point out that 1% of a very large table can still be a lot of disk
> I/O that is avoided (especially if it's random I/O that's avoided)
Sure, but I think that trying to avoid it will be costly in other ways
- you'll be streaming a huge volume of data through some auxiliary
process, which will have to apply some algorithm that's very different
from the one we use today. The reality is that I think there's little
evidence that the way we do ANALYZE now is too expensive. It's
typically very cheap and works very well. It's a bit annoying when it
fires off in the middle of a giant data load, so we might need to
change the time of it a little, but if there's a problem with the
operation itself being too costly, this is the first I'm hearing of
it. We've actually worked *really* hard to make it cheap.
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
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