public void addCustomRollUpMask(final CustomRollUpMask customRollUpMask) {
        if (customRollUpMasks == null) {
            customRollUpMasks = new HashSet<>();
        }

        customRollUpMasks.add(customRollUpMask);
    }
Cache the positions of the fields.
stroom-core-shared/src/main/java/stroom/statistics/impl/hbase/shared/StroomStatsStoreEntityData.java