Github user JamesRTaylor commented on the pull request:
https://github.com/apache/phoenix/commit/48e589773cbf46a10a2c1bd5cf483f2390ae1160#commitcomment-16855409
When a group by is being performed, unless the group by key is made up of
the leading PK columns of a table or index, Phoenix holds in memory each
distinct group by key and the partial aggregation. This will spill to disk
based on phoenix.groupby.maxCacheSize, but performance will drop in this case.
There may be other in memory representations we can use to reduce the memory
footprint. I've filed PHOENIX-2800 for some potential future work. Let me know
if you'd be interested in contributing that.
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