Item::val_xxx() may be called by the server several times at execute time 
 for a single query. Calls to val_xxx() may be very expensive and sometimes
 (count(distinct), sum(distinct), avg(distinct)) not possible.
 To avoid that problem the results of calculation for these aggregate 
 functions are cached so that val_xxx() methods just return the calculated 
 value for the second and subsequent calls.


mysql-test/r/func_group.result:
  Bug #23184: SELECT causes server crash
   - test case
mysql-test/t/func_group.test:
  Bug #23184: SELECT causes server crash
   - test case
sql/item_sum.cc:
  Bug #23184: SELECT causes server crash
   - caching of the aggregate function results so no need to recalculate at val_xxx()
sql/item_sum.h:
  Bug #23184: SELECT causes server crash
   - caching of the aggregate function results so no need to recalculate at val_xxx()