The aggregation on VARCHAR(128) column(number of distinct values is aproximately 31 bln) fails with an obscure error.

ERROR 1815 (HY000): Internal error: TupleAggregateStep::threadedAggregateRowGroups()[24] MCS-2054: Unknown error while aggregation.  

The current implementation of RowAggStorage::increaseSize() can raise RowAggStorage::Data::fMask 4 times before rehashing happens. The guarding check in increaseSize() is too restrictive and fails easily with big numbers in fCurData->fMask and fCurData->fSize(see RowAggStorage::increaseSize() for details).

The suggested solution is to increase the multiplier in the expression:

if (fCurData->fSize * 2 < calcMaxSize(fCurData->fMask + 1))