On Mon, Apr 15, 2013 at 5:33 PM, Tom Lane <t...@sss.pgh.pa.us> wrote:
> Rodrigo Barboza <rodrigombu...@gmail.com> writes:
> > I created a type 'mytype' (an unsigned int) and created an operator class
> > for index.
> > Then I created a table with a column of my type and isnerted 1000
> entries.
> > But no matter how many entries I have in the table, it never uses the
> > index. It always does a seq scan.
>
> > Here is the explain analyze with 1000 entries:
>
> > explain analyze select * from mytable where a > 120::mytype and a <
> > 530::mytype;
>
> >
> ---------------------------------------------------------------------------------------------------
> > Seq Scan on mytable (cost=0.00..19.02 rows=400 width=4) (actual
> > time=0.023..0.229 rows=409 loops=1)
> > Filter: ((a > '120'::mytype) AND (a < '530'::mytpe))
> > Total runtime: 0.297 ms
>
> Using a seqscan to fetch 400 out of 1000 entries is the right thing.
> (The crossover point where an index becomes unhelpful is a lot closer
> to 1% of the table than it is to 40%.)
>
> regards, tom lane
>
You were right Tom, when I did < 200 it used the index.
But I have another question.
I created a implic cast for mytype to bigint.
So when I do the same query it does seq scan, because the column is
transformed into bigint.
Is there a good solution for this?
Here is an examples with the 2 queries.
explain analyze select * from mytable where a < 200::mytype;
QUERY PLAN
--------------------------------------------------------------------------------------------------------------------
Index Scan using mytype_idx on tm32 (cost=0.00..11.66 rows=195 width=4)
(actual time=0.020..0.068 rows=200 loops=1)
Index Cond: (a < '200'::mytype)
Total runtime: 0.111 ms
explain analyze select * from mytable where a < 200;
QUERY PLAN
---------------------------------------------------------------------------------------------------
Seq Scan on mytable (cost=0.00..19.02 rows=334 width=4) (actual
time=0.015..0.215 rows=200 loops=1)
Filter: ((a)::bigint < 200)
Total runtime: 0.238 ms
