Hi Alvin, its a good point and something we have been thinking about for a while. This is something that we definitely need to act on soon.
The challenge is to find a sensible level of detail for what to store, as there are costs in terms of i) system complexity and ii) aggregation performance degradation involved here. For example, if facilities are moved around in administrative boundaries every month, how do you calculate the yearly total for the whole province? My suggestion, as a reasonable trade-off, would be to create snapshots of the organisation unit hierarchy at the end of each year. These snapshots are then used for analytics queries for past data. So for instance if you query today for "last 12 months", it would use the current "live" organisation unit hierarchy for months Jan-Sept 2013, then the "2012 snapshot" hierarchy for months Oct-Dec 2012. On an implementation note, this will fit well the analytics solution where data is partitioned across years - each data partition will have a corresponding org unit hierarchy, making aggregation simple. Lars
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