public String constructEventKey(ComplexEvent event) {
        if (groupByExecutors != null) {
            StringBuilder sb = new StringBuilder();
            for (ExpressionExecutor executor : groupByExecutors) {
                sb.append(executor.execute(event)).append(SiddhiConstants.KEY_DELIMITER);
            }
            return sb.toString();
        } else {
            return null;
        }
    }
generate groupBy key of a streamEvent

@param event complexEvent
@return GroupByKey
modules/siddhi-core/src/main/java/io/siddhi/core/query/selector/GroupByKeyGenerator.java