Litao Wu <litaowu(at)yahoo(dot)com> writes:
> When I included a subquery, the estimated rows (1240)
> is way too high as shown in the following example.
> select * from test
> where scope=(select 10);
The planner sees that as "where scope = <some complicated expression>"
and falls back to a default estimate. It won't simplify a sub-select
to a constant. (Some people consider that a feature ;-).)
The estimate should still be derived from the statistics for the
scope column, but it will just depend on the number of distinct
values for the column and not on the specific comparison constant.
regards, tom lane