From: Bruce Momjian <pgman@candle.pha.pa.us>
Message-Id: <200502250049.j1P0nBK08307@candle.pha.pa.us>
Subject: Re: PostgreSQL ignores my indexes
In-Reply-To: <20050223142526.GA9072@phlogiston.dyndns.org>
To: Andrew Sullivan <ajs@crankycanuck.ca>
Date: Thu, 24 Feb 2005 19:49:11 -0500 (EST)
Cc: pgsql-sql@postgresql.org
MIME-Version: 1.0
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Andrew Sullivan wrote:
> This is probably better on -performance, and is certainly a FAQ. 
> But. . .
> 
> On Wed, Feb 23, 2005 at 03:01:52PM +0100, Thomas Braad Toft wrote:
> > 
> > Table device contains 5285 rows, tmeevent contains 834912 rows.
>                         ^^^^                         ^^^^^^
> 
> > ->  Seq Scan on tmeevent  (cost=0.00..23606.12 rows=834912 width=138)
> > (actual time=0.04..2193.97 rows=834912 loops=1)
>                                   ^^^^^^
> 
> > ->  Seq Scan on device  (cost=0.00..564.85 rows=5285 width=29) (actual
> > time=0.04..25.07 rows=5285 loops=1)
>                         ^^^^
> 
> > Why isn't the planner using my indexes? I tried making them as both rtree
> 
> Because there's no advantage to using an index when you are fetching
> 100% of both tables.  This is the most efficient plan.  Of course,
> it's an open question whether you want to get 100% of both tables. 
> But that's what you're doing, and using the index would be more
> expoensive than this.

Right.  The FAQ addresses this issue.

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