Using the table/data generated in MCOL-1396 on a single server with 64GB of RAM do the following query:

select * from mcol1396 group by a,b,c;

You will get the error:

ERROR 1815 (HY000) at line 1: Internal error: IDB-2003: Aggregation/Distinct memory limit is exceeded

The ExeMgr memory is not freed and grows even more with the next query. I haven't looked into whether or not it is eventually freed but it means it is easy to hit OOM.