From: Richard Huxton <dev@archonet.com>
To: Giorgio Valoti <giorgio_v@mac.com>
Cc: pgsql-performance@postgresql.org
Subject: Re: Query Plan choice with timestamps
Date: Thu, 07 Aug 2008 13:36:18 +0100
Message-ID: <489AEC42.5080007@archonet.com> (raw)
In-Reply-To: <15821866-9564-40E8-BC12-52ADD6D6EEA5@mac.com>
References: <321506D5-4400-470C-909E-C7700D49A540@mac.com>
<489AB3EB.8040903@archonet.com>
<15821866-9564-40E8-BC12-52ADD6D6EEA5@mac.com>
Giorgio Valoti wrote:
>
> On 07/ago/08, at 10:35, Richard Huxton wrote:
>
>> Giorgio Valoti wrote:
>>> Hi, I have a timestamptz field that I want to use with a query, but I
>>> don’t need the full timestamp resolution, so I’ve created a
>>> day_trunc(timestamptz) immutable function which I’ll use with the
>>> query and with a new index:
>>> logs=> create index test_idx on blackbox (day_trunc(ts));
>>> However, the query plan doesn’t use the index:
>>
>> Does it use it ever? e.g. with
>> SELECT * FROM blackbox WHERE day_trunk(ts) = '...'
>
> It’s used:
[snip]
OK - so the index is working.
If you disable seq-scans before running the query, does it use it then?
SET enable_seqscan = off;
> logs=>explain select count(*) from blackbox group by day_trunc(ts) order
> by day_trunc(ts);
> QUERY PLAN
>
------------------------------------------------------------------------------------------
>
> GroupAggregate (cost=98431.58..119773.92 rows=74226 width=8)
In particular:
1. Is the estimated cost more or less than 119773.92?
2. How does that match the actual time taken?
--
Richard Huxton
Archonet Ltd
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To: pgsql-performance@postgresql.org
Cc: dev@archonet.com, giorgio_v@mac.com
Subject: Re: Query Plan choice with timestamps
In-Reply-To: <489AEC42.5080007@archonet.com>
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