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Coincindences

I had another of those odd timing events today that make me think that Larry Ellison has access to a time machine. I found (yet another example of a) bug that had been reported on MoS just a few days before it appeared on an instance I was running. How is it possible that someone keeps doing things that I’m doing, but just a few days before I do them !

For no good reason I happened to browse through a load of background trace files on an 11.2.0.4 instance and found the following in an “m000″ file:

Cache anomaly

Just a quick heads-up for anyone who likes to play around with the Keep and Recycle caches.

In 11g Oracle introduced the option for serial direct path reads for tablescans on tables that was sufficiently large – which meant more than the small_table_threshold – provided the table wasn’t already sufficient well cached.  (The rules mean that the choice of mechanism can appear to be a little random in the production environment for tables that are near the threshold size – but if you try testing by doing “alter system flush buffer_cache” you find that you always get direct path reads in testing.)

Cache anomaly

Just a quick heads-up for anyone who likes to play around with the Keep and Recycle caches.

In 11g Oracle introduced the option for serial direct path reads for tablescans on tables that was sufficiently large – which meant more than the small_table_threshold – provided the table wasn’t already sufficient well cached.  (The rules mean that the choice of mechanism can appear to be a little random in the production environment for tables that are near the threshold size – but if you try testing by doing “alter system flush buffer_cache” you find that you always get direct path reads in testing.)

Easy – Oops.

A question came up on OTN today asking for suggestions on how to enforce uniqueness on a pair of columns only when the second column was not null. There’s an easy and obvious solution – but I decided to clone the OP’s example and check that I’d typed my definition up before posting it; and the result came as a bit of a surprise. Here’s a demo script (not using the OP’s table):

Easy – Oops.

A question came up on OTN today asking for suggestions on how to enforce uniqueness on a pair of columns only when the second column was not null. There’s an easy and obvious solution – but I decided to clone the OP’s example and check that I’d typed my definition up before posting it; and the result came as a bit of a surprise. Here’s a demo script (not using the OP’s table):

RLS bug

RLS – row level security, aka VPD (virtual private database) or FGAC (fine grained access control) has a critical bug in 11g. The bug is unpublished, but gets mentioned in various other documents, so can be identified as: Bug: 7828323 “SYS_CONTEXTS RETURNS WRONG VALUE WITH SHARED_CONTEXT_SENSITIVE”

The title tells you nearly everything you need to know – if you’ve declared a security policy as context_sensitive or shared_context_sensitive then a change to the context ought to result in the associated predicate function being called to generate a new security predicate the next time the policy becomes relevant. Thanks to bug 7828323 this doesn’t always happen – so queries can return the wrong set of results.

RLS bug

RLS – row level security, aka VPD (virtual private database) or FGAC (fine grained access control) has a critical bug in 11g. The bug is unpublished, but gets mentioned in various other documents, so can be identified as: Bug: 7828323 “SYS_CONTEXTS RETURNS WRONG VALUE WITH SHARED_CONTEXT_SENSITIVE”

The title tells you nearly everything you need to know – if you’ve declared a security policy as context_sensitive or shared_context_sensitive then a change to the context ought to result in the associated predicate function being called to generate a new security predicate the next time the policy becomes relevant. Thanks to bug 7828323 this doesn’t always happen – so queries can return the wrong set of results.

Shrink Space

Here’s a lovely effect looking at v$lock (on 11.2.0.4)

Subquery Anomaly

Here’s an oddity that appeared on the OTN database forum last night:

We have this query in our application which works fine in 9i but fails in 11gR2 (on Exadata) giving an “ORA-00937: not a single-group group function” error….

… The subquery is selecting a column and it doesn’t have a group by clause at all. I am not sure how is this even working in 9i. I always thought that on a simple query using an aggregate function (without any analytic functions / clause), we cannot select a column without having that column in the group by clause. So, how 11g behaves was not a surprise but surprised to see how 9i behaves. Can someone explain this behaviour?

Flashback Fail ?

Sitting in an airport, waiting for a plane, I decided to read a note (pdf) about Flashback data archive written by Beat Ramseier from Trivadis.  I’d got about three quarters of the way through it when I paused for thought and figured out that on the typical database implementation something nasty is going to happen after approximately 3 years and 9 months.  Can you guess why ?