If you changed the case of a table name, this means you had to change whatever was generating the SQL since mixed or lower case requires table or column names to be surrounded by double quotes.

So, you changed code and didn't test it. Yup, definitely Oracle's fault.

Except that's not what I'm referring to. As with any cost based optimiser, it's only as good as the statistics it has to work with, and if you've finally carefully tuned your database with an appropriate histogram on a critical table, and found that after an upgrade they "tweaked" the CBO to fix a statistics calculation then you are right back to looking at explain plans to fix your performance issue.

Sometimes you just aren't going to pick up this sort of thing in testing as you'll only know about it under full production load. Their CBO is pretty powerful, but by and large it's often difficult to know why it makes certain decisions. And that's even when you use their full suite of profiling tools!

Having a poorly thought out query planner as the cornerstone of their product is a sign that the product is not that good.

In all fairness, I can't really blame Oracle on this one. That's a fairly well documented issue, because Oracle places the hash value of each query into the shared SQL pool, and if CURSOR_SHARING is set to exact and you've stopped the query from aging out then Oracle will see it as a different query.

That's not even really a CBO issue. But I hear your pain :-)