Re: Sequential scan evaluating function for each row, seemingly needlessly

Bryce Nesbitt <bryce2(at)obviously(dot)com> writes:
> On psql 8.3.9, I ran a limited query limited to 5 results. There was a
> moderately expensive function call
> which I expected to be called 5 times, but was apparently called for
> each row of the sequential scan. Why?

Given the plan:

> Limit (cost=19654.53..19654.54 rows=5 width=12) (actual
> time=10001.976..10001.990 rows=5 loops=1)
> -> Sort (cost=19654.53..19826.16 rows=68651 width=12) (actual
> time=10001.972..10001.976 rows=5 loops=1)
> Sort Key: add_date
> Sort Method: top-N heapsort Memory: 25kB
> -> Seq Scan on extractq (cost=0.00..18514.26 rows=68651
> width=12) (actual time=19.145..9770.689 rows=73550 loops=1)
> Total runtime: 10002.150 ms
> (6 rows)

any interesting work is going to be done at the seqscan level. Sort
just sorts, and Limit just limits; neither do any user-defined
calculations. So yeah, your functions got run for every row of the
table. (This isn't totally a PG aberration, btw: if you read the SQL
spec closely you'll discover that ORDER BY is defined to happen after
any calculations specified in the SELECT list.)

You could try something like

select my_expensive_function(...), etc, etc from
(select * from some-tables order by foo limit n) ss;

where the inner select list just pulls the columns you'll need in
the outer calculations.

regards, tom lane