The server wide stats feature, which is currently disabled by default, runs queries directly on the sakai_event and sakai_session tables. This generates charts for historical data, and takes an extremely long time to return, often resulting in a Wicket pagemap timeout.
This also means that institutions cannot clean out the sakai_session or sakai_event tables as they will lose this historical reporting ability. The performance impact of querying the millions of records in these tables hasn't been as major an issue as the timeouts have been, often rendering the functionality useless. We have experienced this in production.
Finally, it runs SQL directly via the Sakai SqlService, and this is hardcoded and only written for MySQL.
This ticket will track work to reimplement the server wide stats feature to use the event listening capabilities to store this historical data. There will also be a conversion to take historical data and put it into the new structure.