private String getIndex(OperatorNode<ExpressionOperator> operatorNode) {
        String index = fetchFieldName(operatorNode);
        String expanded = indexNameExpander.expand(index);
        Preconditions.checkArgument(indexFactsSession.isIndex(expanded), "Field '%s' does not exist.", expanded);
        return indexFactsSession.getCanonicName(index);
    }
The sorting specified in the YQL+ query last parsed.

@return a Sorting instance or null
container-search/src/main/java/com/yahoo/search/yql/YqlParser.java