A visitor that modifies the logical plan (if necessary) for union-onschema
functionality. It runs logical plan validator so that the correct schema
of its inputs is available. It inserts foreach statements in its input
if the input operator schema does not match the schema created by
merging all input schemas.
Migrated from the old UnionOnSchemaSetter class.
Fields inherited from class org.apache.pig.newplan.PlanVisitor