In the last weeks I noticed an increased number of cases where Oracle starts to pick notoriously bad plans for some statements on widely different systems (Exadata, ODA and AIX).
I haven’t seen that much before with our applications, so I wonder if this is related to a recent patch (the systems seems to have different releases/Versions). Or maybe the default analyser settings have changed?
In one particular case a primary (unique) index is not used for a simple primary key lookup but instead it seems to use some kind of index fast scan. I thought adaptive coursor sharing is actually not used for that kind of simple query.