[Alter] The rollup index's row number is same as base table.

[morningman]

Describe the bug
After creating a rollup with less key column, we found that the row number of this rollup
is just same as the base table.

To Reproduce

1. create a table with column k1, k2, k3, v1(sum)
2. load some data.
3. create rollup with column k1, v1(sum)

after rollup finished, the num rows of rollup is same as base table's.

Expected behavior
Because the number of key columns in rollup is less than base table, the data
should be more aggregate in rollup, so the row num of the rollup should be less than
base table.