Infrastructure

Jonathan Lewis's picture

Bigfiles

It’s always the combinations that catch you out.

Bigfile tablespaces have their uses – especially in big systems

Materialized views have their users – especially in big systems

There’s absolutely no reason why the two technologies should interfere with each other … until you find a bug !

Running an example, stripped to the bare minimum, and doing a couple of things that I personally don’t like doing, on 11.2.0.4:

Jonathan Lewis's picture

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 The bug is unpublished, but gets mentioned in various other documents, so can be identified as: Bug: 7828323 “SYS_CONTEXTS RETURNS WRONG VALUE WITH SHARED_CONTEXT_SENSITIVE”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.

Jonathan Lewis's picture

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 The bug is unpublished, but gets mentioned in various other documents, so can be identified as: Bug: 7828323 “SYS_CONTEXTS RETURNS WRONG VALUE WITH SHARED_CONTEXT_SENSITIVE”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.

Jonathan Lewis's picture

Temporary Segments

A question came up on oracle-l recently about identifying which temporary segment in v$tempseg_usage is associated with which global temporary table. Here’s my answer:

v$tempseg_usage is a synonym for v$sort_usage – which is the view that I still tend to think of first.

Jonathan Lewis's picture

Temporary Segments

A question came up on oracle-l recently about identifying which temporary segment in v$tempseg_usage is associated with which global temporary table. Here’s my answer:

v$tempseg_usage is a synonym for v$sort_usage – which is the view that I still tend to think of first.

Jonathan Lewis's picture

12c Temporary

Just one of those little snippets to cover something new and remind of something old. A single session can now have three (or more) temporary tablespaces in use at the same time for different reasons.

Jonathan Lewis's picture

12c Temporary

Just one of those little snippets to cover something new and remind of something old. A single session can now have three (or more) temporary tablespaces in use at the same time for different reasons.

Jonathan Lewis's picture

Duplicate database

One of the people attending my seminar in Munich last week has emailed me some details about a nasty little surprise you may get if you’re thinking about TSPITR (tablespace point in time recovery), and happen to have a few materialized views in your database.

You might have wanted to use the “duplicate target database”  from rman with the “skip tablespace” option to create a duplicate database, excluding tablespaces that you’re not interested in, if you needed to recover a particular tablespace (or set of tablespaces) to a specific point in time.  (Once you’ve done that you can extract the data that you want and then discard the duplicate database). But there’s a requirement for recovery is that the set of tablespaces should be “self-contained”; but what does “self-contained” mean ?

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Auto Sample Size

In the past I have enthused mightily about the benefits of the approximate NDV mechanism and the benefit of using auto_sample_size to collect statistics in 11g; however, as so often happens with Oracle features, there’s a down-side or boundary condition, or edge case. I’ve already picked this up once as an addendum to an earlier blog note on virtual stats, which linked to an article on OTN describing how the time taken to collect stats on a table increased dramatically after the addition of an index – where the index had this definition:

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Indexing LOBs

Many years ago, possibly when most sites were still using Oracle 8i, a possible solution to a particular customer problem was to create a function-based index on a CLOB column using the dbms_lob.getlength() function call. I can’t find the notes explaining why this was necessary (I usually have some sort of clue – such as the client name – in the script, but in this case all I had was a comment that “the manuals say you can’t do this, but it works provided you wrap the dbms_lob call inside a deterministic function”).

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