Re: Query optimization question - Mailing list pgsql-general

Hi,

avoid min(col) or max(col), use "ORDER BY col DESC|ASC LIMIT 1" instead.
It is faster on postgresql. I dunno why, but the archives shuold give
the answer ;-)

Hope this helps,
-tb


On Mon, Nov 18, 2002 at 05:49:28PM -0800, Daniel S. Myers wrote:
> Hi,
>     I’m working on a large-scale simulation study in biology, and
> I’m using PostgreSQL as the hub of a distributed computing system.
> Essentially, I have a single table containing 2.7 million rows, each of
> which represents a work unit to be processed (think SETI@Home). Among
> other fields, each row in the table contains a unique ID and a status,
> which is one of WAIT (not yet processed) OUT (processing) or DONE
> (completed). To dispatch a unit to a client, I’d like to pick a row with
> status = WAIT, mark it as OUT, and return the values. The problem I have
> is that finding a row takes a really long time (~22s on a 2-way PIII-700
> running Linux 2.4.19). My SQL looks like: select min(id) from analyses
> where status=’WAIT’. I have indexes on the id field and the status
> field, but an explain shows that they’re not being used—I’m assuming
> that the optimizer has decided that there’s no point in using the index,
> since it doesn’t sufficiently limit the scope of the search. Is there
> some optimization that I’m missing (I’ve tried indexes on status/id and
> id/status as well as the VACUUM and CLUSTER), or will I have to do
> something more than the naïve approach? (We’re actually sampling 2700
> points 1000 times, so I can use replicates_executed counters in each row
> and have a separate results table if I have to; it’d just be nice to
> keep things simple if possible).
>
> Thanks in advance,
> Daniel
> dmyers@pomona.edu
>
> P.S.: I’ve included the code to create the analyses table below.
>
> create table analyses (
>     id serial,
>     kind char(5) not null check (kind in ('FAST', 'SLOW')),
>     host varchar(255) references hosts(hostname),
>     dispatched timestamp,
>     received timestamp,
>     status char(5) not null check (status in ('WAIT', 'OUT', 'DONE'))
> default 'WAIT',
>
>     /* Plus a bunch of parameters for the simulation... */
> );
>
>
>
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--
Thomas Beutin                             tb@laokoon.IN-Berlin.DE
Beam me up, Scotty. There is no intelligent live down in Redmond.