"Pavel Stehule" <[EMAIL PROTECTED]> writes: > It's strange, so correlated subqueries is faster, but it has much higher cost:
In the nestloop plan, the estimated cost for the indexscan is discounted based on the knowledge that it'll be executed repeatedly: > -> Index Scan using fxxx on history t2 > (cost=0.00..0.31 rows=1 width=11) (actual time=0.017..0.087 rows=40 > loops=5003) In the subplan case that doesn't happen: > -> Index Scan using fxxx on history (cost=0.00..8.27 > rows=1 width=4) (actual time=0.018..0.086 rows=40 loops=5003) > Index Cond: (((product)::text = ($0)::text) AND (id <= $1)) Had the same discount been applied then the estimated costs would be pretty nearly in line with reality, if I did the math right. It'd be nice to do better but I'm not sure how; at the time that we create plans for sub-queries we don't really have any way to know how often they'll be called by the upper query. regards, tom lane -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers