Re: bad execution plan for subselects containing windowing-function

Andreas Kretschmer <akretschmer(at)spamfence(dot)net> writes:
> Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> I see no bug here. Your second query asks for a much more complicated
>> computation, it's not surprising it takes longer.

> But sorry, I disagree. It is the same query with the same result. I can't see
> how the queries should return different results.

In the first query

select id, avg(value) over (partition by value) from values where id = 50 order by id;

the avg() calculations are being done over only rows with id = 50. In
the second query

select * from (select id, avg(value) over (partition by value) from values order by id) foo where id = 50;

they are being done over all rows. In this particular example you
happen to get the same result, but that's just because "avg(foo) over
partition by foo" is a dumb example --- it will necessarily just yield
identically foo. In more realistic computations the results would be
different.

regards, tom lane