From: Tobias Brox <tobias@nordicbet.com>
To: pgsql-performance@postgresql.org
Subject: max/min and index usage
Date: Wed, 6 Dec 2006 04:01:56 +0100
Message-ID: <20061206030156.GA29255@oppetid.no> (raw)
We're using 8.1 - I thought such a construct was safe in pg 8.1:
select max(indexed_value) from huge_table;
while earlier we had to use:
select indexed_value from huge_table order by indexed_value desc limit 1;
seems like I was wrong:
mydb=> explain analyze select indexed_value1 from mytable order by indexed_value1 desc limit 1;
QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------------------------------------------------
Limit (cost=0.00..1.96 rows=1 width=4) (actual time=0.115..0.117 rows=1 loops=1)
-> Index Scan Backward using index1 on mytable (cost=0.00..23890756.52 rows=12164924 width=4) (actual time=0.111..0.111 rows=1 loops=1)
Total runtime: 0.162 ms
(3 rows)
mydb=> explain analyze select indexed_value2 from mytable order by indexed_value2 desc limit 1;
QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------------------------------------------
Limit (cost=0.00..0.04 rows=1 width=4) (actual time=0.128..0.130 rows=1 loops=1)
-> Index Scan Backward using index2 on mytable (cost=0.00..428231.16 rows=12164924 width=4) (actual time=0.124..0.124 rows=1 loops=1)
Total runtime: 0.160 ms
(3 rows)
mydb=> explain analyze select max(indexed_value2) from mytable;
QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Result (cost=0.04..0.05 rows=1 width=0) (actual time=11652.138..11652.139 rows=1 loops=1)
InitPlan
-> Limit (cost=0.00..0.04 rows=1 width=4) (actual time=11652.122..11652.124 rows=1 loops=1)
-> Index Scan Backward using index2 on mytable (cost=0.00..428231.16 rows=12164924 width=4) (actual time=11652.117..11652.117 rows=1 loops=1)
Filter: (indexed_value2 IS NOT NULL)
Total runtime: 11652.200 ms
(6 rows)
mydb=> explain analyze select max(indexed_value1) from mytable;
QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Result (cost=1.96..1.97 rows=1 width=0) (actual time=713.780..713.781 rows=1 loops=1)
InitPlan
-> Limit (cost=0.00..1.96 rows=1 width=4) (actual time=713.767..713.770 rows=1 loops=1)
-> Index Scan Backward using index1 on mytable (cost=0.00..23890756.52 rows=12164924 width=4) (actual time=713.764..713.764 rows=1 loops=1)
Filter: (indexed_value1 IS NOT NULL)
Total runtime: 713.861 ms
(6 rows)
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To: pgsql-performance@postgresql.org
Cc: tobias@nordicbet.com
Subject: Re: max/min and index usage
In-Reply-To: <20061206030156.GA29255@oppetid.no>
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