One of the problems of functions is that the optimizer generally doesn’t have any idea on how a predicate based on function(col) might affect the cardinality. However, the optimizer group are constantly refining the algorithms to cover an increasing number of special cases more accurately. This is a good thing, of course – but it does mean that you might be unlucky on an upgrade where a better cardinality estimate leads to a less efficient execution plan. Consider for example the simple query (where d1 is column of type date):
select *
from t1
where nvl(d1,to_date('01-01-1900','dd-mm-yyyy')) < sysdate
Now, there are many cases in many versions of Oracle, where the optimizer will appear to calculate the cardinality of
nvl(columnX,{constant}) operator {constant}
as if it were: