public String getMappedOrder() {
    return mappedOrder.toString();
  }
General factory method to map two operator keys.
Two keys are considered to be equal
- if parent operator's <code>parentColExprMap</code> has an entry with the operator key column name
- and that entry value has the same index as the operator key column index.
- and both key columns has the same ordering
- and both key columns has the same null ordering

Ex.: op1: a, b, c, d
     op2: a, b, e
     result: a, b

     opKeys: Column[_col0], Column[_col1], Column[_col2], Column[_col3]
     parentKeys: Column[KEY._col0], Column[KEY._col1], Column[KEY._col4]
     parentColExprMap: {_col0 -&gt; Column[KEY._col0]}, {_col1 -&gt; Column[KEY._col1]}, {_col4 -&gt; Column[KEY._col4]}

Column ordering and null ordering is given by a string where each character represents a column order/null order.
Ex.: a ASC NULLS FIRST, b DESC NULLS LAST, c ASC NULLS LAST -&gt; order="+-+", null order="azz"

When <code>parentColExprMap</code> is null this method falls back to
{@link #map(List, String, String, List, String, String)}.

@param opKeys {@link List} of {@link ExprNodeDesc}. contains the operator's key columns
@param opOrder operator's key column ordering in {@link String} format
@param opNullOrder operator's key column null ordering in {@link String} format
@param parentKeys {@link List} of {@link ExprNodeDesc}. contains the parent operator's key columns
@param parentColExprMap {@link Map} of {@link String} -&gt; {@link ExprNodeDesc}.
                                   contains parent operator's key column name {@link ExprNodeDesc} mapping
@param parentOrder parent operator's key column ordering in {@link String} format
@param parentNullOrder parent operator's key column null ordering in {@link String} format
@return {@link CommonKeyPrefix} object containing the common key prefix of the mapped operators.
ql/src/java/org/apache/hadoop/hive/ql/optimizer/topnkey/CommonKeyPrefix.java