Have just upgraded from 12.5.3 to 15.5/EBF 18376.
One of the queries which used to run in less than 1 minutes in ASE 12.5, needs more than 2 hours to run on 15.5. Did update index stat in 15.5 no effect.
We therefore decided to set the compatibility mode on, the response time became again <1 minute.
Indeed in version 15.5 the generated plan is a monster and besides indices are not used, while all is optimized in 12.5.
I have noticed that the DML are much faster in version 15, I mean from 15.03 to 15.7 all inclussive, but heavy queries are to be used with caution in ASE 15.
Is this true or we have some wrong doing in our shop? Help is needed.