Thread: max/min and index usage

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..0.111rows=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..0.124rows=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..11652.117rows=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..713.764rows=1 loops=1) 
                 Filter: (indexed_value1 IS NOT NULL)
 Total runtime: 713.861 ms
(6 rows)

[Tobias Brox - Wed at 04:01:56AM +0100]
> 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:

The difference is all about those NULL values ... those columns are quite
sparsely populated in the table.  The second query gives NULL, which is
not much useful :-)

However, I made a partial index to solve this problem - this query is
able to use the partial index:

  select indexed_value from huge_table where indexed_value is not NULL
  order by indexed_value desc limit 1;

while this one is not:

  select max(indexed_value) from huge_table;

I guess this is a bug? :-)

Tobias Brox <tobias@nordicbet.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

[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.