Re: Query planner is using wrong index.

On fös, 2006-04-07 at 00:01 +1000, Brian Herlihy wrote:
> --- Ragnar <gnari(at)hive(dot)is> wrote:
>
> > On fim, 2006-04-06 at 19:27 +1000, Brian Herlihy wrote:
> >
> > > Yes, the primary key is far better. I gave it the ultimate test - I
> > dropped
> > > the (p2, p3) index. It's blindingly fast when using the PK,
> >
> > I have problems understanding exactly how an index on
> > (p1,p2,p3) can be faster than and index on (p2,p3) for
> > a query not involving p1.

> db# explain analyze select * from t WHERE p1 = 'a' and p2 = 'uk.altavista.com'
> AND p3 = 'web/results?itag=&q=&kgs=&kls=';

this is different from what you said earlier. in your
original post you showed a problem query without any
reference to p1 in the WHERE clause. this confused me.

> Index Scan using p2_p3_idx on t (cost=0.00..6.02 rows=1 width=102) (actual
> time=2793.247..2793.247 rows=0 loops=1)
> Index Cond: (((p2)::text = 'uk.altavista.com'::text) AND ((p3)::text =
> 'web/results?itag=&q=&kgs=&kls='::text))
> Filter: ((p1)::text = 'a'::text)
> Total runtime: 2793.303 ms
> (4 rows)

try to add an ORDER BY clause:

explain analyze
select * from t
WHERE p1 = 'a'
and p2 = 'uk.altavista.com'
AND p3 = 'web/results?itag=&q=&kgs=&kls='
ORDER BY p1,p2,p3;

this might push the planner into using the primary key

gnari