If group by key contains constant columns, the constant columns should be removed from group by key

[windtalker]

Enhancement

Consider the following cases

1. select count(*), id from test where id = 1 group by id
2. select count(*), id, value from test where id = 1 group by id, value

If there is no implict cast in expr id = 1, then id should be a constant column after filter, it can be removed from the group by keys, that is the optimizer should rewrite the sql to

1. select count(*), first_row(id) from test where id = 1
2. select count(*), first_row(id), value from test where id = 1 group by value

[winoros]

For the first query, we cannot remove the group-by directly because the result is different for an empty table whether it's a group-by agg or just a global agg.

[dash12653]

It seems that an RBO rule needs to be added. I will try to handle this issue.