For performance reason, we want to remove constant column from groupingExprs.
For example:
`select sum(T.A) from T group by T.B, 'xyz'` is equivalent to `select sum(T.A) from T group by T.B`
We can remove constant column `abc` from groupingExprs.
But there is an exception when all groupingExpr are constant
For example:
sql1: `select 'abc' from t group by 'abc'`
is not equivalent to
sql2: `select 'abc' from t`
sql3: `select 'abc', sum(a) from t group by 'abc'`
is not equivalent to
sql4: `select 1, sum(a) from t`
(when t is empty, sql3 returns 0 tuple, sql4 return 1 tuple)
We need to keep some constant columns if all groupingExpr are constant.
Consider sql5 `select a from (select "abc" as a, 'def' as b) T group by b, a;`
if the constant column `a` is in select list, this column should not be removed.
sql5 is transformed to
sql6 `select a from (select "abc" as a, 'def' as b) T group by a;`
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