Partitioning and constraint exclusion

Hello ,

I have a parent table and 6 child tables (partitions). The child tables
have check constraints defined in the form
CHECK (myuid in (123,456,..)).
myuid is bigint, the constraints for the 6 child tables are definitely
mutually exclusive. The number of values in the list ranges from 2-10 for 5
of the child tables. For the 6th child table, the list is 2500+ elements.
When I try explain/explain analyze for even a simple query like

select * from parent where myuid in (123,456,789)

the child table with 2500+ elements gets always scanned. I have an index on
the column and that does get used. But why doesn't the planner just use
constraint exclusion and not go for the index scan? Anyone faced a similar
issue?

Thanks,
Jayadevan