On Thu, Dec 13, 2012 at 7:36 PM, Andrew Dunstan <andrew@xxxxxxxxxxxx> wrote: > On 12/13/2012 05:12 PM, AI Rumman wrote: >> >> Why does the number of rows are different in actual and estimated? >> > > > Isn't that in the nature of estimates? An estimate is a heuristic guess at > the number of rows it will find for the given query or part of a query. It's > not uncommon for estimates to be out by several orders of magnitude. > Guaranteeing estimates within bounded accuracy and in a given short amount > of time (you don't want your planning time to overwhelm your execution time) > isn't possible. Although this kind of difference could be indeed a problem: > Nested Loop (cost=18560.97..26864.83 rows=24871 width=535) (actual time=1335.157..8492.414 rows=157953 loops=1) > -> Hash Left Join (cost=18560.97..26518.91 rows=116 width=454) (actual time=1335.117..6996.585 rows=205418 loops=1) It usually is due to some unrecognized correlation between the joined tables. And it looks like it all may be starting to go south here: > -> Hash Join (cost=9337.97..18115.71 rows=34489 width=244) (actual time=418.054..1156.453 rows=205420 loops=1) > Hash Cond: (customerdetails.customerid = entity.id) -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
