This task is similar to MCOL-4543, but for subqueries involving a UNION. I.e., If query Q1 is of the form:

SELECT count(c2) FROM (SELECT * FROM t1 UNION ALL SELECT * FROM t1)q;

Assuming t1 here contains 10 columns c1, c2, ... , c10. We build an ineffective RowGroup in ExeMgr of the form (1, c2_value1, 1, 1, 1, 1, 1, 1, 1, 1). The objective here is to remove all non-referenced columns from the end, until the first referenced column is encountered, i.e. trim down the RowGroup to (1, c2_value1).