public RexNode get(int index) {
            final int pos = posList.get(index);
            return rexBuilder.makeInputRef(child, pos);
          }
Creates a relational expression that projects the given fields of the
input.

<p>Optimizes if the fields are the identity projection.

@param relBuilder RelBuilder
@param child Input relational expression
@param posList Source of each projected field
@return Relational expression that projects given fields
ql/src/java/org/apache/hadoop/hive/ql/optimizer/calcite/HiveRelOptUtil.java