max/min and index usage

Lists: pgsql-performance
From: Tobias Brox <tobias(at)nordicbet(dot)com>
To: pgsql-performance(at)postgresql(dot)org
Subject: max/min and index usage
Date: 2006-12-06 03:01:56
Message-ID: 20061206030156.GA29255@oppetid.no
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Lists: pgsql-performance

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)


From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Tobias Brox <tobias(at)nordicbet(dot)com>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: max/min and index usage
Date: 2006-12-06 03:29:53
Message-ID: 17474.1165375793@sss.pgh.pa.us
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Tobias Brox <tobias(at)nordicbet(dot)com> writes:
> 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;

These are not actually exactly the same thing. In particular, I suppose
your table contains a lot of nulls?

regards, tom lane


From: Tobias Brox <tobias(at)nordicbet(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Tobias Brox <tobias(at)nordicbet(dot)com>, pgsql-performance(at)postgresql(dot)org
Subject: Re: max/min and index usage
Date: 2006-12-06 03:35:50
Message-ID: 20061206033550.GB26712@oppetid.no
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[Tom Lane - Tue at 10:29:53PM -0500]
> These are not actually exactly the same thing. In particular, I suppose
> your table contains a lot of nulls?

Yes; I'm sorry I was a bit quick with the first posting.