=?ISO-8859-1?Q?Christian_Schr=F6der?= <cs@xxxxxxxxx> writes: > if I want to find all records from a table that don't have a matching > record in another table there are at least two ways to do it: Using a > left outer join or using a subselect. I always thought that the planner > would create identical plans for both approaches, but actually they are > quite different which leads to a bad performance in one case. No, they're not the same; NOT IN has different semantics for nulls. > Another interesting thing: If table "a" contains only 400,000 rows > (instead of 500,000) the query planner decides to use a hashed subplan > and performance is fine again: You're probably at the threshold where it doesn't think the hashtable would fit in work_mem. regards, tom lane -- Sent via pgsql-general mailing list (pgsql-general@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
