The symptom is that the same join query yield different results with MJ and NLJ/HJ. Here is a repro:
--- create table t1(a int);create table t2(b int); insert into t1 values(10); insert into t2 values(2); analyze t1; analyze t2; set enable_mergejoin=on; set enable_nestloop=off; set enable_hashjoin=off; explain analyze select a, b from t1 left join t2 on coalesce(a, 1) = coalesce(b,1) where (coalesce(b,1))>0 set enable_mergejoin=off; set enable_nestloop=on; set enable_hashjoin=off; explain analyze select a, b from t1 left join t2 on coalesce(a, 1) = coalesce(b,1) where (coalesce(b,1))>0 --- A possible explanation is that in fix_join_expr_mutator(), we optimize with the case that if child node already compute an expression then upper node shall reuse it. In MJ, as coalesce() already computed in sort node, thus the NULL is directly used for ExecQual(>0) for join filter. If we take out this optimization the problem is solved but may looks like an overkill. What's a better fix? Thanks, Qingqing -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
