Incremental rebuild of materialized view having aggregate and stored by iceberg

Currently incremental rebuild of materialized view stored by iceberg which definition query contains aggregate operator is transformed to an insert overwrite statement which contains a union operator if the source tables contains insert operations only. One branch of the union scans the view the other produces the delta.

This can be improved further: transform the statement to a multi insert statement representing a merge statement to insert new aggregations and update existing.