Обсуждение: aggregate functions, COUNT

I'm currently using a SELECT count(*) when all I really want to know is
if 1 or more records exist.  Is there a standard way to just find out if
a record exists?  If not, is there a way to avoid iterating over all the
records by writing an aggregate function?  Given what I've read of how
they work, I don't see how to make the function return before parsing
all the results anyway, am I wrong here?

Thanks for any help, or 2x4s,
Kevin Way

-- 
Kevin Way <kevin.way@overtone.org>
http://www.overtone.org/

On Tue, 2 Oct 2001, Kevin Way wrote:

> I'm currently using a SELECT count(*) when all I really want to know is
> if 1 or more records exist.  Is there a standard way to just find out if
> a record exists?  If not, is there a way to avoid iterating over all the
> records by writing an aggregate function?  Given what I've read of how
> they work, I don't see how to make the function return before parsing
> all the results anyway, am I wrong here?

I think you could use EXISTS for that,
select EXISTS (<query>); should give a true/false on whether the 
query returned any rows.  I'm not sure if it stops after one row
or not, but if it doesn't you can add a limit 1 to the query.

Stephan Szabo <sszabo@megazone23.bigpanda.com> writes:
> I think you could use EXISTS for that,
> select EXISTS (<query>); should give a true/false on whether the 
> query returned any rows.  I'm not sure if it stops after one row
> or not, but if it doesn't you can add a limit 1 to the query.

Yes it does stop after one row; and furthermore, the planner knows to
generate a fast-start plan for it.  (Or at least it's supposed to
... hmm, this seems to be broken in current sources ...)  Anyway,
there's no need for LIMIT 1 inside an EXISTS, because the planner
assumes that automatically.
        regards, tom lane

* Tom Lane <tgl@sss.pgh.pa.us> [02-10-01 18:02]:
> Stephan Szabo <sszabo@megazone23.bigpanda.com> writes:
> > I think you could use EXISTS for that,
> > select EXISTS (<query>); should give a true/false on whether the 
> > query returned any rows.  I'm not sure if it stops after one row
> > or not, but if it doesn't you can add a limit 1 to the query.
> 
> Yes it does stop after one row; and furthermore, the planner knows to
> generate a fast-start plan for it.  (Or at least it's supposed to
> ... hmm, this seems to be broken in current sources ...)  Anyway,
> there's no need for LIMIT 1 inside an EXISTS, because the planner
> assumes that automatically.

Thank you muchly, I did some profiling and SELECT EXISTS is indeed
exactly what I wanted.  
       -Kevin Way

-- 
Kevin Way <kevin.way@overtone.org>
http://www.overtone.org/