I thought I’d try to spend some of today catching up on old comments – first the easier ones, then the outstanding questions on Oracle Core.
The very first one I looked at was about pushing predicates, and the specific comment prompted me to jot down this little note about the 10053 trace file (the CBO trace).
Here’s an interesting anomaly from the OTN database forum – PL/SQL track. It’s a single row delete from a table that does a huge number of db block gets and (in the first example shown) physical reads. Here’s the supplied output with autotrace enabled:
I’ve written about dynamic sampling in the past, but here’s a little wrinkle that’s easy to miss. How do you get the optimizer to work out the correct cardinality for a query like (the table creation statement follows the query):
select count(*) from t1 where n1 = n2 ; create table t1 as with generator as ( select --+ materialize rownum id from dual connect by level <= 1e4 ) select mod(rownum, 1000) n1, mod(rownum, 1000) n2 from generator v1, generator v2 where rownum <= 1e6 ;
If you’re running 11g and can changed the code there are a couple of easy options – adding a virtual column, or applying extended stats and then modifying the SQL accordingly would be appropriate.
Some time ago – actually a few years ago – I wrote a note about the hint /*+ gather_plan_statistics */ making some informal comments about the implementation and relevant hidden parameters. I’ve recently discovered a couple of notes from Alexander Anokhin describing the feature in far more detail and describing some of the misleading side effects of the implementaiton. There are two parts (so far): part 1 and part 2.
Dominic Brooks published a note recently about some very nasty SQL – originally thinking that it was displaying a run-time problem due to the extreme number of copies of the lnnvl() function the optimizer had produced. In fact it turned out to be a parse-time problem rather than a run-time problem, but when I first read Dominic’s note I was sufficiently surprised that I decided to try modelling the query.
Unfortunately the query had more than 1,000 predicates, (OR’ed together) and some of them included in-lists. Clearly, writing this up by hand wasn’t going to be a good idea, so I wrote a script to generate both the data, and the query, as follows – first a table to query:
Here’s a quick and dirty script to create a procedure (in the SYS schema – so be careful) to check the Hakan Factor for an object. If you’re not familiar with the Hakan Factor, it’s the value that gets set when you use the command “alter table minimize records_per_block;”.
I was prompted to publish this note by an item on the OTN SQL forum describing a problem with partition exchange with a table when there were bitmap indexes in place and the table had been changed to have some extra columns added. (Problem as yet unresolved as I publish).
If you start playing with the Hakan Factor, you’ll find that there are some odd little bugs in what gets stored and how it gets used. (SQL updated to use bitand() to reflect comments below and Karsten Spang’s blog note; also edited following a comment on OTN to show the rest of the spare1 flag bits)
The problem of slow queries on v$lock just came up again on the OTN database forum, so I thought I’d better push out a post that’s been hanging around on my blog for the last few months. This is actually mentioned in MOS in note 1328789.1: “Query Against v$lock Run from OEM Performs Slowly” which points out that it is basically a problem of bad statistics and all you have to do is collect the stats.
For your entertainment – there’s nothing up my sleeves, this was a simple cut-n-paste after real-time typing with no tricks:
20:39:51 SQL> create table t1 (t1 timestamp); Table created. 20:39:55 SQL> insert into t1 values(systimestamp); 1 row created. 20:39:59 SQL> select t1 - systimestamp from t1; T1-SYSTIMESTAMP --------------------------------------------------------------------------- +000000000 04:59:50.680620 1 row selected. 20:40:08 SQL>
My laptop runs Oracle so quickly that it took only 4 seconds for 5 hours to elapse !
126.96.36.199 on 64-bit Linux – the client is running with TZ=EST5EDT, while the server is running UK Time (currently BST (GMT+1))
Here’s a deadlock graph that might cause a little confusion:
You’ve probably seen questions on the internet occasionally about finding out how frequently an object has been modified. The question is a little ambiguous – does it mean how much change has occurred, or how many DML statements have been executed; either may be an interesting measure. Of course, Oracle gave us a method of answering the first question a long time ago: v$segstat (or v$segment_statistics if you don’t mind doing the join) and the resulting content in the AWR or Statspack reports: