private void executeOrderBy() {
        for (ExpressionNode node : orderByExp) {
            node.prepare();
            node.execute();
        }
    }
Merges the content of the given group <b>into</b> this. When this function returns, make sure to call
{@link #postMerge(java.util.List, int, int)}.

@param firstLevel   The first level to merge.
@param currentLevel The current level.
@param rhs          The group to merge with.
searchlib/src/main/java/com/yahoo/searchlib/aggregation/Group.java