Thread: subquery in limit

(just as an example):
select * from test order by a limit (select count(*)*0.9 from test);

is not doable in postgresql. Someone recently asked on IRC about,  
"SELECT TOP 90 PERCENT" type of query in m$sql.
Any ideas how should this be approach in psql. I ask here, because you  
guys probably can tell why the first query won't work (subquery is not  
allowed as limit's argument, why?).

cheers.

-- 
Grzegorz Jaskiewicz
gj@pointblue.com.pl

I have no idea why you can't do a subquery in the limit but you can
reference a function:

create table test as select * from pg_tables;

create or replace function fn_count(p_sql varchar) returns int as
$$
declare v_count int;
begin execute p_sql into v_count; return v_count;
end;
$$
language 'plpgsql' security definer;


select * from test limit fn_count('select round(count(*)*0.9) from
test');

And I'm sure someone will point out a more efficient way to write my
function without using pl/pgsql.  :)


Jon


> -----Original Message-----
> From: pgsql-hackers-owner@postgresql.org [mailto:pgsql-hackers-
> owner@postgresql.org] On Behalf Of Grzegorz Jaskiewicz
> Sent: Friday, February 15, 2008 5:35 AM
> To: pgsql-hackers@postgresql.org
> Subject: [HACKERS] subquery in limit
>
> (just as an example):
> select * from test order by a limit (select count(*)*0.9 from test);
>
> is not doable in postgresql. Someone recently asked on IRC about,
> "SELECT TOP 90 PERCENT" type of query in m$sql.
> Any ideas how should this be approach in psql. I ask here, because you
> guys probably can tell why the first query won't work (subquery is not
> allowed as limit's argument, why?).
>
> cheers.
>
> --
> Grzegorz Jaskiewicz
> gj@pointblue.com.pl
>
>
>
>
> ---------------------------(end of
broadcast)---------------------------
> TIP 3: Have you checked our extensive FAQ?
>
>                http://www.postgresql.org/docs/faq

"Roberts, Jon" <Jon.Roberts@asurion.com> writes:
> I have no idea why you can't do a subquery in the limit

It hasn't seemed worth putting any effort into --- AFAIR this is the
first time anyone's even inquired about it.  As you say, you can always
use a function.

> And I'm sure someone will point out a more efficient way to write my
> function without using pl/pgsql.  :)

Only that it doesn't seem a particularly bright idea to use SECURITY
DEFINER for a function that will execute any arbitrary caller-provided
SQL ...
        regards, tom lane

> "Roberts, Jon" <Jon.Roberts@asurion.com> writes:
> > I have no idea why you can't do a subquery in the limit
>
> It hasn't seemed worth putting any effort into --- AFAIR this is the
> first time anyone's even inquired about it.  As you say, you can
always
> use a function.
>
> > And I'm sure someone will point out a more efficient way to write my
> > function without using pl/pgsql.  :)
>
> Only that it doesn't seem a particularly bright idea to use SECURITY
> DEFINER for a function that will execute any arbitrary caller-provided
> SQL ...
>

LOL!  I knew something in my code would trigger a response.  :)



Jon