Re: SELECT from a set of values really slow?

test=> insert into bench (id,data) select id, 'text_item_'||id::text from
dummy where id<=100000 order by id;
INSERT 0 100001
test=> CREATE INDEX bench_data_index ON bench (data);
CREATE INDEX
test=> explain select * from bench where data = 'test_item_1';
Index Scan using bench_data_index on bench (cost=0.00..1478.85 rows=501
width=36)
Index Cond: (data = 'test_item_1'::text)
(2 lignes)

test=> explain select * from bench where data in ( 'test_item_1',
'test_item_2' );
Seq Scan on bench (cost=0.00..2190.01 rows=998 width=36)
Filter: ((data = 'test_item_1'::text) OR (data = 'test_item_2'::text))
(2 lignes)

test=> vacuum analyze bench;
VACUUM
test=> explain select * from bench where data in ( 'test_item_1',
'test_item_2' );
Index Scan using bench_data_index, bench_data_index on bench
(cost=0.00..7.91 rows=2 width=22)
Index Cond: ((data = 'test_item_1'::text) OR (data =
'test_item_2'::text))
(2 lignes)

=> once you analyze, it works...

***********************************************

With 1 item :

test=>explain analyze select * from bench where data in ( 'test_item_1' );
Total runtime: 0.127 ms

With 11 items :

test=>explain analyze select * from bench where data in ( 'test_item_1',
'test_item_2', 'test_item_55', 'test_item_64', 'test_item_1005',
'test_item_78541', 'test_item_96521', 'test_item_8574', 'test_item_89652',
'test_item_14527', 'test_item_48652' );
Total runtime: 0.352 ms

***********************************************

With a Join... see on psql-performance

On Sun, 10 Oct 2004 16:00:10 -0400, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> Tim Smith <reply_in_group(at)mouse-potato(dot)com> writes:
>> SELECT id FROM bench WHERE data IN ('X', 'Y')
>
>> To my surprise, when I tried this trick with PostgreSQL, it did not
>> speed things up. In fact, it *massively* slowed down--it only is
>> getting 13 selects in 3 seconds, searching for two at a time.
>
>> What's going on here?
>
> Likely it's switching from index to sequential scan because of a poor
> estimate of how many rows will be returned. Have you ever ANALYZEd
> the test table? Without either ANALYZE stats or a unique index,
> the planner will certainly not think that the column is unique.
>
> regards, tom lane
>
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