From: Bruno Wolff III <bruno@wolff.to>
To: Josh Berkus <josh@agliodbs.com>
Cc: pgsql-sql@postgresql.org
Subject: Re: Two Index Questions
Date: Fri, 19 Jul 2002 12:56:38 -0500
Message-ID: <20020719175638.GA14836@wolff.to> (raw)
In-Reply-To: <200207191028.18547.josh@agliodbs.com>
References: <200207190933.59283.josh@agliodbs.com>
<20020719170000.GB13657@wolff.to>
<200207191028.18547.josh@agliodbs.com>
On Fri, Jul 19, 2002 at 10:28:18 -0700,
Josh Berkus <josh@agliodbs.com> wrote:
>
> Bruno,
>
> > > I have two questions for my "Adventures in PostgreSQL" article reasearch:
> > >
> > > Multi-Column Indexes and GROUP BY:
> > > Q: If you group a table by multiple colums, e.g.
> > > SELECT t1.A, t1.B, t1.C, MAX(t1.G)
> > > FROM t1
> > > GROUP BY t1.A, t1.B, t1.C
> > > Then would a multi-column index on A, B, C be faster than seperate
> indexes
> > > on A, B and C? I've run a few tests, but I don't have enough data in the
> > > seperate tables to really get a feel for the difference.
> >
> > If there are lots of G entries for fixed As, Bs and Cs, then another option
> > would be to have an index on all 4 tables and use a subquery with a limit 1
> > clause to get the row with the max G value for any A, B and C.
>
> We're talking about only one table, here. Not four.
Typo. I meant to say columns. The issue is that max doesn't use an index,
but if there are a lot of different values of G for a given A, B and C,
it may be better to use an index then to search through the applicable
rows to find the maximum.
> I generally try to avoid using LIMIT, as it is a non-SQL92 extension. Also,
> LIMIT in subqueries might someday be disallowed as it interferes with the
> fundmentally unordered nature of subqueries.
I can see not using it because it is nonstandard. I don't think it would be
disabled for the reason you have given. The documentation makes it pretty
clear that you need to use order by if you want a particular tuple.
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To: pgsql-sql@postgresql.org
Cc: bruno@wolff.to, josh@agliodbs.com
Subject: Re: Two Index Questions
In-Reply-To: <20020719175638.GA14836@wolff.to>
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