From: Jim C. Nasby <jnasby@pervasive.com>
To: Craig A. James <cjames@modgraph-usa.com>
Cc: pgsql-performance@postgresql.org
Subject: Re: Optimizer: limit not taken into account
Date: Wed, 17 May 2006 14:20:34 -0500
Message-ID: <20060517192034.GW26212@pervasive.com> (raw)
In-Reply-To: <446B474C.2050803@modgraph-usa.com>
References: <44699C6C.80302@tweakers.net>
<4469D173.50406@sun.com>
<4469D986.6090300@tweakers.net>
<4469F55A.7040004@sun.com>
<446A1520.2020601@tweakers.net>
<446B0409.7050902@sun.com>
<446B0ED2.5030909@tweakers.net>
<446B474C.2050803@modgraph-usa.com>
On Wed, May 17, 2006 at 08:54:52AM -0700, Craig A. James wrote:
> Here's a "corner case" that might interest someone. It tripped up one of
> our programmers.
>
> We have a table with > 10 million rows. The ID column is indexed, the
> table has been vacuum/analyzed. Compare these two queries:
>
> select * from tbl where id >= 10000000 limit 1;
> select * from tbl where id >= 10000000 order by id limit 1;
>
> The first takes 4 seconds, and uses a full table scan. The second takes 32
> msec and uses the index. Details are below.
>
> I understand why the planner makes the choices it does -- the "id >
> 10000000" isn't very selective and under normal circumstances a full table
> scan is probably the right choice. But the "limit 1" apparently doesn't
> alter the planner's strategy at all. We were surprised by this.
Is it really not very selective? If there's 10000000 rows in the table,
and id starts at 1 with very few gaps, then >= 10000000 should actually
be very selective...
Also, I hope you understand there's a big difference between a limit
query that does and doesn't have an order by.
--
Jim C. Nasby, Sr. Engineering Consultant jnasby@pervasive.com
Pervasive Software http://pervasive.com work: 512-231-6117
vcard: http://jim.nasby.net/pervasive.vcf cell: 512-569-9461
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To: pgsql-performance@postgresql.org
Cc: jnasby@pervasive.com, cjames@modgraph-usa.com
Subject: Re: Optimizer: limit not taken into account
In-Reply-To: <20060517192034.GW26212@pervasive.com>
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