From: Shaun Thomas <sthomas@peak6.com>
To: Kevin Grittner <Kevin.Grittner@wicourts.gov>
Cc: CS DBA <cs_dba@consistentstate.com>
Cc: pgsql-performance@postgresql.org
Subject: Re: Problem query
Date: Thu, 2 Jun 2011 12:31:58 -0500
Message-ID: <4DE7C90E.3020408@peak6.com> (raw)
In-Reply-To: <4DE770E6020000250003E01C@gw.wicourts.gov>
References: <4DE69D92.5040407@consistentstate.com>
<4DE66AF8020000250003DFB6@gw.wicourts.gov>
<4DE6BD11.6060603@consistentstate.com>
<4DE74E06020000250003DFE6@gw.wicourts.gov>
<4DE7AD6E.3020604@peak6.com>
<4DE768E9020000250003E002@gw.wicourts.gov>
<4DE770E6020000250003E01C@gw.wicourts.gov>
On 06/02/2011 11:15 AM, Kevin Grittner wrote:
> They all gave the same result, of course, and they all used a seq
> scan..
And they all will. I created a test table with a bunch of
generate_series and emulated 200 unique matches of column1 and column2,
on a table with a mere 1-million rows (5000 for each of column3). And no
matter what index combination I used, it always did a sequence scan...
even when I indexed every column and indexed column3 descending.
But here's the thing. I turned off sequence scans to force index scans,
and it got 2-3x slower. But is that really surprising? Without a proper
where exclusion, it has to probe every occurrence... also known as a
loose index scan, which PostgreSQL doesn't have (yet).
And... this is horrifying, but:
WITH RECURSIVE t1 AS (
SELECT min(f.tds_cx_ind) AS tds_cx_ind
FROM max_xtrv_st_t f
UNION ALL
SELECT (SELECT min(tds_cx_ind)
FROM max_xtrv_st_t f
WHERE f.tds_cx_ind > t1.tds_cx_ind)
FROM t1
WHERE t1.tds_cx_ind IS NOT NULL
), t2 AS (
SELECT min(f.cxs_ind_2) AS cxs_ind_2
FROM max_xtrv_st_t f
UNION ALL
SELECT (SELECT min(cxs_ind_2)
FROM max_xtrv_st_t f
WHERE f.cxs_ind_2 > t2.cxs_ind_2)
FROM t2
WHERE t2.cxs_ind_2 IS NOT NULL
)
SELECT t1.tds_cx_ind, t2.cxs_ind_2 FROM t1, t2
WHERE t1.tds_cx_ind IS NOT NULL
AND t2.cxs_ind_2 IS NOT NULL;
It works on my test, but might not be what OP wants. It's a cross
product of the two unique column sets, and it's possible it represents
combinations that don't exist. But I suppose a late EXISTS pass could
solve that problem.
I assume there's an easier way to do that. In either case, when is PG
getting loose index scans? ;)
--
Shaun Thomas
OptionsHouse | 141 W. Jackson Blvd. | Suite 800 | Chicago IL, 60604
312-676-8870
sthomas@peak6.com
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To: pgsql-performance@postgresql.org
Cc: sthomas@peak6.com, Kevin.Grittner@wicourts.gov, cs_dba@consistentstate.com
Subject: Re: Problem query
In-Reply-To: <4DE7C90E.3020408@peak6.com>
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