You’ve probably heard about adaptive cursor sharing, and possibly you’ve wondered why you haven’t seen it happening very often on your production systems (assuming you’re running a modern version of Oracle). Here’s a little bug/fix that may explain the non-appearance.
MOS Doc ID 9532657.8 Adaptive cursor sharing ignores SELECTs which are not fully fetched.
This bug is confirmed in 220.127.116.11, and fixed in 18.104.22.168. The problem is that the ACS code doesn’t process the statistical information from the cursor unless the cursor reaches “end of fetch” – i.e. if you don’t select all the data in your query, Oracle doesn’t consider the statistics of that execution when deciding whether or not to re-optimise a statement.
It’s quite possible, of course, for an OLTP system, and particularly a web-based system, to execute a number of that allow the user to fetch data one “page” at a time, and stop before fetching all the data – so this bug (or limitation, perhaps) means that some critical statements in your application may never be re-optimized. If this is the case, and you know that you have some such statements that should generate multiple plans, then you could add the hint /*+ bind_aware */ to the SQL.
Upgrade woes: as ever, when a bug is fixed, it’s possible that a few people will suffer from unfortunate side-effects. In the case of this bug, Oracle may start to re-optimize and generated multiple child cursors for SQL statements that (from your perspective) didn’t need the extra work. If you’re very unlucky this may have an undesirable impact on execution performance, and library cache activity.
Thanks to Leonid Roodnitsky for sending me a note about this bug after attending one of my tutorial days last month.