Re: constraints in query plans

On Sun, 2006-10-15 at 20:36 -0700, Jeremy Drake wrote:
> I just tried that,
> CREATE INDEX test_domain_k_x1_x2_mp ON test_domain (k, x1, x2, mp);
>
> and dropped the others. That actually works properly.
> jeremyd=# explain analyze select * from test_domain where k = 1255 and mp;
> QUERY PLAN
> ----------------------------------------------------------------------------------------------------------------------------------
> Bitmap Heap Scan on test_domain (cost=5.37..237.21 rows=66 width=17) (actual time=0.115..0.707 rows=132 loops=1)
> Recheck Cond: (k = 1255)
> Filter: mp
> -> Bitmap Index Scan on test_domain_k_x1_x2_mp (cost=0.00..5.37 rows=66 width=0) (actual time=0.081..0.081 rows=132 loops=1)
> Index Cond: ((k = 1255) AND (mp = true))
> Total runtime: 1.137 ms
> (6 rows)
>
> I thought I had to refer to all of the columns in order for this to work,
> that I could not skip some in the middle, but it seems to work.
>

As long as "k=1255" is selective enough, the index is useful. That's
because k is the first item in the index key.

Regards,
Jeff Davis