Thread: Postgres8: subselect and optimizer/planner

Hi,

I am fairly new to EXPLAIN, butl working on it. ;-)
I have a few slow running queries I am trying to optimize.

First thing I wonder: I sometimes (lazy) add a subselect to queries.
A stupid example to clearify what I mean:

SELECT U.userid, U.username,
(SELECT G.groupname FROM tblgroup WHERE (G.userid=U.userid)) AS ingroup
FROM tbluser WHERE (bla..bla...);

Will this approach be slower than a regular join?

I mean, will this construct 'force' a repetitive query for each result,
or will Postgres8 see my clumpsy construct, and make a join of it
internally?

Or is my question too general and is the answer 'it depends'?

I found a lot of queries I wrote like that in earlier projects, and I
wonder if I should fix them.
Thanks for any insights!

Regards,
Erwin Moller

--



-------------------
Erwin Moller
Darwine BV

Groenendaal 25f
3011 SK Rotterdam
tel 010-2133996
-------------------

Erwin Moller wrote:
>
> SELECT U.userid, U.username,
> (SELECT G.groupname FROM tblgroup WHERE (G.userid=U.userid)) AS ingroup

typo, that should be 'tblgroup as G' of course.

> FROM tbluser WHERE (bla..bla...);


-------------------
Erwin Moller
Darwine BV

Groenendaal 25f
3011 SK Rotterdam
tel 010-2133996
-------------------

Erwin Moller <erwin@darwine.nl> writes:
> SELECT U.userid, U.username,
> (SELECT G.groupname FROM tblgroup WHERE (G.userid=U.userid)) AS ingroup
> FROM tbluser WHERE (bla..bla...);

> Will this approach be slower than a regular join?

Probably; it's unlikely to be faster anyway.  The best plan you'll get
from this is equivalent to a nestloop with inner indexscan on
tblgroup.userid.  Now that might be the best plan anyway, or it might
not --- if you are selecting many rows from ingroup it's likely to suck.

> Or is my question too general and is the answer 'it depends'?

The only way I could see for this way to win would be if a nestloop is
actually the fastest plan, but the planner misestimates and decides to
use merge or hash join instead.  Which could happen :-(

            regards, tom lane