Re: SQL 'in' vs join.

mlw wrote:
>
> Why is a "select * from table1 where field in (select field from table2
> where condition )"
>
> is so dramatically bad compared to:
>
> "select * from table1, table2 where table1.field = table2.field and
> condition"
>
> I can't understand why the first query isn't optimized better than the
> second one. The 'in' query forces a full table scan (it shouldn't) and
> the second one uses the indexes. Does anyone know why?

Its not done yet, and probably hsomewhat difficult to do in a general
fashion

> I know I am no SQL guru, but my gut tells me that the 'in' operator
> should be far more efficient than a join.
>
> Here are the actual queries:
>
> cdinfo=# explain select trackid from zsong where muzenbr in (select
> muzenbr from ztitles where title = 'Mulan') ;

try

explain
select trackid
from zsong
where muzenbr in (
select muzenbr
from ztitles
where title = 'Mulan'
and ztitles.muzenbr=zsong.muzenbr
);

this should hint the current optimizer to do the right thing;

-----------------
Hannu