This just in from OTN Database Forum – a surprising little bug with “group by elimination” exclusive to 12c.
alter session set nls_date_format='dd-Mon-yyyy hh24:mi:ss';
select
/* optimizer_features_enable('12.1.0.1')*/
trunc (ts,'DD') ts1, sum(fieldb) fieldb
from (
select
ts, max(fieldb) fieldb
from (
select trunc(sysdate) - 1/24 ts, 1 fieldb from dual
union all
select trunc(sysdate) - 2/24 ts, 2 fieldb from dual
union all
select trunc(sysdate) - 3/24 ts, 3 fieldb from dual
union all
select trunc(sysdate) - 4/24 ts, 4 fieldb from dual
union all
select trunc(sysdate) - 5/24 ts, 5 fieldb from dual
)
group by ts
)
group by trunc (ts,'DD')
/
You might expect to get one row as the answer – but this is the result I got, with the execution plan pulled from memory:
