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Troubleshooting

Deadlocks

Here’s a deadlock graph that might cause a little confusion:

DML Tracking

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:

Index Selectivity

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:

Open Cursors

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:

Duplicate indexes ?

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 11.1.0.7 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 ;) )

Deadlock Detection

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):

Transactions 2

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.

Transactions

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 ?

Losing it

The example I gave last week showing how a SORT operation in an execution plan might include the work of resolving function calls in your SQL and might, therefore, be reporting much higher resource utilisation than expected reminded me of some problems I’ve had with gaps in execution plans in the past. So I thought I’d give a little demonstration of the way in which the completeness of execution plans can develop over time.

We’ll start with the same two tables I had in last week’s demo.

Sorting

Here’s a little quirk of execution plans that came up recently on the Oak Table network. If you call a function in a query, and do some sorting with the results, where does the work of calling the function get reported in the execution plan if you trace the query or look at the in-memory rowsource execution stats. Let’s take a look at a simple example:

create table t1
as
select
	rownum 			id,
	lpad(rownum,200)	padding
from	all_objects
where	rownum <= 2500
;

create table t2
as
select	* from t1
;

-- collect stats

create or replace function f (i_target in number)
return number
as
	m_target	number;
begin
	select max(id) into m_target from t1 where id <= i_target;
	return m_target;
end;
/