Bryce Nesbitt <bry...@obviously.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 -- Sent via pgsql-sql mailing list (pgsql-sql@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-sql