Re: why my query is not using index??

Am Mittwoch, 6. Oktober 2004 09:31 schrieben Sie:
> postgres=# explain ANALYZE select * from test where today < '2004-01-01';
> QUERY PLAN
>------------------------- Seq Scan on test (cost=0.00..19.51 rows=334
> width=44) (actual
> time=0.545..2.429 rows=721 loops=1)
> Filter: (today < '2004-01-01 00:00:00'::timestamp without time zone)
> Total runtime: 3.072 ms
> (3 rows)
>
> postgres=# explain ANALYZE select * from test where today > '2003-01-01'
> and today < '2004-01-01';
> QUERY
> PLAN
> --------------------------------------------------------------- Index
> Scan using idx_today on test (cost=0.00..18.89 rows=6 width=44) (actual
> time=0.055..1.098 rows=365 loops=1)
> Index Cond: ((today > '2003-01-01 00:00:00'::timestamp without time
> zone) AND (today < '2004-01-01 00:00:00'::timestamp without time zone))
> Total runtime: 1.471 ms
> (3 rows)
>
> hello
>
> I was expected 1st query should using index, but it doesn't
> 2nd query doing perfect as you see.

postgres uses a seq scan if its faster. In your case postgres seems to know
that most of your rows have a date < 2004-01-01 and so doesn't need to
consult the index if it has to read every page anyway. seq scan can be faster
on small tables. try (in psql) "SET enable_seqscan TO off;" before running
your query and see how postgres plans it without using seq scan.

janning