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 !
188.8.131.52 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:
Here’s a summary of a recent posting on OTN:
I have two indexes (REFNO, REFTYPESEQNO) and (REFNO,TMSTAMP,REFTYPESEQNO). When I run the following query the optimizer uses the second index rather than the first index – which is an exact match for the predicates, unless I hint it otherwise:
Here’s a little detail that appeared in 11gR2 that may help you answer questions about open cursors. Oracle has added a “cursor type” column to the view v$open_cursor, so you can now see which cursors have been held open because of the pl/sql cursor cache, which have been held by the session cursor cache, and various other reasons why Oracle may take a short-cut when you fire a piece of SQL at it.
The following is the output showing the state of a particular session just after it has started up in SQL*Plus and called a PL/SQL procedure to run a simple count:
I don’t think this is likely to happen on a production system (until 12c) – but look what you can do if you try hard enough:
1 select 2 index_name, column_name from user_ind_columns 3 where 4 table_name = 'T1' 5 order by 6* index_name , column_position SQL> / INDEX_NAME COLUMN_NAME -------------------- -------------------- T1_I1 N1 V1 T1_I2 N1 V1 4 rows selected.
That’s a straight cut-n-paste from an Oracle 184.108.40.206 SQL*Plus session. (You can tell I typed it in real time because I missed the return before the FROM, and couldn’t be bothered to go back and do it again ;) )
By some strange coincidence, the “London Bus” effect perhaps, there have been three posts on the OTN database forum in the last couple of days relating to deadlocks; and they have prompted me to indulge in a little rant about the myth of Oracle and deadlock detection; it’s the one that goes:
“Oracle detects and resolves deadlocks automatically.”
Oracle may detect deadlocks automatically, but it doesn’t resolve them, it simply reports them (by raising error ORA-00060 and rolling back one statement) then leaves the deadlocked sessions stuck until the session that received the report resolves the problem or an external agent resolves the problem.
Consider the following example (which, I have to admit, I wrote without access to a live instance):
Here’s a little follow-on from Friday’s posting. I’ll start it off as a quiz, and follow up tomorrow with an explanation of the results (though someone will probably have given the correct solution by then anyway).
I have a simple heap table t1(id number(6,0), n1 number, v1 varchar2(10), padding varchar2(100)). The primary key is the id column, and the table holds 3,000 rows where id takes the values from 1 to 3,000. There are no other indexes. (I’d show you the code, but I don’t want to make it too easy to run the code, I want you to try to work it out in your heads).
I run the following pl/sql block.
It’s very easy to get a lot of information from an AWR (or Statspack) report – provided you remember what all the numbers represent. From time to time I find that someone asks me a question about some statistic and my mind goes completely blank about the exact interpretation; but fortunately it’s always possible to cross check because so many of the statistics are cross-linked. Here’s an example of a brief mental block I ran into a few days ago – I thought I knew the answer, but realised that I wasn’t 100% sure that my memory was correct:
In this Load Profile (for an AWR report of 60.25 minutes), what does that Transactions figure actually represent ?