@Override
    public String toString() {
        return "FindEventCriteria [period=" + period + ", statisticName=" + statisticName + ", filterTermsTree="
                + filterTermsTree + ", rolledUpFieldNames=" + rolledUpFieldNames + "]";
    }
Return the {@link FilterTermsTree} object on this criteria

@return The filter tree, may be null if not filter is defined.
stroom-statistics/stroom-statistics-impl-sql/src/main/java/stroom/statistics/impl/sql/search/FindEventCriteria.java