To: Thom Brown <thombrown@gmail.com>
cc: pgsql-performance <pgsql-performance@postgresql.org>
Subject: Re: Wildly inaccurate query plan 
In-reply-to: <AANLkTil3OC1HqAsbeElDL6imkp9Yq5kA9smS3vwHsguC@mail.gmail.com> 
References: <AANLkTil3OC1HqAsbeElDL6imkp9Yq5kA9smS3vwHsguC@mail.gmail.com>
Comments: In-reply-to Thom Brown <thombrown@gmail.com>
	message dated "Fri, 28 May 2010 19:27:08 +0100"
Date: Fri, 28 May 2010 14:54:25 -0400
Message-ID: <4337.1275072865@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

Thom Brown <thombrown@gmail.com> writes:
> I get this:

> Limit  (cost=0.00..316895.11 rows=400 width=211) (actual
> time=3.880..1368.936 rows=400 loops=1)
>    ->  GroupAggregate  (cost=0.00..41843621.95 rows=52817 width=211)
> (actual time=3.872..1367.048 rows=400 loops=1)
>          ->  Index Scan using "binaryID_2576_idx" on parts_2576
> (cost=0.00..41683754.21 rows=10578624 width=211) (actual
> time=0.284..130.756 rows=19954 loops=1)
>                Index Cond: (("binaryID")::text >
> '1082fa89fe499741b8271f9c92136f44'::text)
>  Total runtime: 1370.140 ms

> The first thing which strikes me is how the GroupAggregate step shows
> it got the 400 rows which matches the limit, but it estimated 52,817
> rows.  Shouldn't it have already known it would be 400?

No.  Rowcount estimates are always in terms of what the node would emit
if allowed to run to completion.  Likewise cost.  In this case both the
indexscan and the groupagg are terminated early once they satisfy the
limit.  The planner is expecting this which is why the estimated cost
for the limit node is way less than those for its inputs.

That looks like a perfectly reasonable plan from here, though it would
probably not get chosen with a larger limit or no limit at all, since
the ultimate costs are pretty large.  Essentially this is a fast-start
plan rather than a lowest-total-cost plan, and that looks like the
best bet for a small limit value.

			regards, tom lane