@Override
  public RelNode trim(RelBuilder relBuilder, RelNode root) {
    return trim(relBuilder, root, null, null);
  }
Trims unused fields from a relational expression.

<p>We presume that all fields of the relational expression are wanted by
its consumer, so only trim fields that are not used within the tree.

@param root Root node of relational expression
@return Trimmed relational expression
ql/src/java/org/apache/hadoop/hive/ql/optimizer/calcite/rules/HiveRelFieldTrimmer.java