MCOL-4543 Subquery optimization.

For a query of the form:

SELECT COUNT(c2) FROM (SELECT * FROM t1) q;

where t1 contains 10 columns c1, c2, ... , c10.

We currently create an intermediate RowGroup in ExeMgr with
a row of the form (1, c2_value1, 1, 1, 1, 1, 1, 1, 1, 1), i.e.
for all the columns of the subquery which are not referenced in
the outer query, we substitute a constant value, which is wasteful.

With this optimization, we are trimming the RowGroup to a row
of the form (1, c2_value1). This can have non-trivial query
execution time improvements if the subquery contains large number
of columns (such as a "select *" on a very wide table) and the outer
query is only referencing a subset of these columns with lower
index values from the subquery (as an example, c1 or c2 above).
That is, the current limitation of this optimization is we are not
removing those non-referenced subquery columns (c1 in the query above)
which are to the left of a referenced column.

dbcon/execplan/calpontselectexecutionplan.h

@@ -360,6 +360,14 @@ public:
         return fColumnMap;
     }
 
+    /** column map
+     * all the columns appeared on query
+     */
+    ColumnMap& columnMap()
+    {
+        return fColumnMap;
+    }
+
     /** assign the static fColMap to non-static fColumnMap. map-wise copy */
     void columnMap (const ColumnMap& columnMap);