Comments on: Lock table followed by DDL?

By: orainternals

orainternals — Tue, 04 Aug 2009 14:08:07 +0000

Hi Saravanan
Thanks for reading my blog. I had a mistake in that part of the script, Sorry. I was testing something else and accidentally must have introduced ‘wait 300′ in the ddl statement. I have corrected it and your sql need to be modified as below:
(BTW, sample is a keyword and it is probably not a good idea to use for table name).
declare
begin
— lock table in exclusive mode
execute immediate ‘lock table t1 in exclusive mode ‘;
dbms_output.put_line (‘**** Table locked. Dropping column ****’ ) ;
— If we lock it, drop the column
execute immediate ‘alter table t1 drop column n5 ‘;
exception
when others then
dbms_output.put_line(‘ No luck ‘);
dbms_output.put_line(‘ SQL code’||sqlcode||’,'|| ‘error ‘||sqlerrm );
end;

Cheers
Riyaj

By: Saravanan

Saravanan — Tue, 04 Aug 2009 04:57:49 +0000

hai friend,
i tried the following PL/SQL when droping a column in lock mode.but it show the ERROR.
SQL> ed
Wrote file afiedt.buf

1 declare
2 begin
3 dbms_output.put_line (‘1′) ;
4 — lock table in exclusive mode and wait for 5 minutes
5 execute immediate ‘lock table sample in exclusive mode wait 60′;
6 dbms_output.put_line (‘2′) ;
7 dbms_output.put_line (‘**** Table locked. Dropping column ****’ ) ;
8 dbms_output.put_line (‘3′) ;
9 — If we lock it, drop the column
10 execute immediate ‘alter table sample add address varchar2(20)’;
11 exception
12 when others then
13 dbms_output.put_line(‘ No luck ‘);
14 dbms_output.put_line(‘ SQL code’||sqlcode||’,'|| ‘error ‘||sqlerrm );
15* end;
SQL> /
1
No luck
SQL code-933,error ORA-00933: SQL command not properly ended

PL/SQL procedure successfully completed.

By: orainternals

orainternals — Sun, 15 Mar 2009 17:43:53 +0000

Mark
I have corrected original comments.

By: Mark Bobak

Mark Bobak — Fri, 13 Mar 2009 16:19:21 +0000

Note: In the comments above, where I said DDL, I meant DML.

By: orainternals

orainternals — Fri, 13 Mar 2009 15:14:58 +0000

Hi Mark
Thanks for visiting my blog and thanks for the idea.
Cheers
Riyaj

By: Mark Bobak

Mark Bobak — Fri, 13 Mar 2009 06:32:48 +0000

Hi Riyaj,

Well, ok, better late than never.

I remember that talk we had some time ago, and the investigation.

Once other solution, if 11g is not yet an option, is available, but would require modification of application code. If that’s an option, you can code DBMS_LOCK to take a user defined enqueue in shared mode before doing DML on the table, and call it from normal application operation. When you want to add column, drop column, whatever, you can call DBMS_LOCK on your user defined enqueue with exclusive mode. This will queue behind the shared mode locks held by the application. Eventually, the app will release those locks, your session will acquire exclusive lock, and then you can proceed w/ drop column, add column, whatever. As long as all DML on the table is happening from the application where code always takes the user defined enqueue first, this should work. When you’re done w/ the add/drop column, release the enqueue, and the application will start running again.

Hope that helps,

-Mark

By: orainternals

orainternals — Fri, 13 Feb 2009 01:28:47 +0000

Hi Hrishy
Thank you for reading my blog. Of course, there are more than one way to do that operation and that entry was related DDL and implicit commit.
Hope you got that job.

Cheers
Riyaj

By: hrishy

hrishy — Mon, 09 Feb 2009 17:29:38 +0000

Coincidence i was asked this question in a interview and the same exact sceanario where the database is very busy.I mentioned my choice as dbms_redefination wundering why you didnt opt for the dbms_redefination way

By: orainternals

orainternals — Tue, 20 Jan 2009 16:44:59 +0000

Hello Robert
Thanks for reading my blog.
Original script was supposed to drop and add few columns. I used ‘drop’ part of the script to make my point. Obviously didn’t catch that comment discrepancy. I have updated that.
About ’sleep’, yeah, there are more than one way to accomplish that goal

Cheers
Riyaj

By: Robert Klemme

Robert Klemme — Tue, 20 Jan 2009 08:55:51 +0000

Another worrying observation that would make a good explanation for “the code block did not do what we wanted”:

dbms_output.put_line (‘**** Table locked. Adding column ****’ ) ;
— If we lock it, drop the column
execute immediate ‘alter table t1 drop column n5 ‘;

The output talks about adding but the DDL is actually a DROP COLUMN… Btw, this also seems to be a bit inconsistent in your article.

Another thing that made me wonder: why did you do the 10 second sleep? If the application is OLTP and the table is heavily used, I’d rather loop without sleep because IMHO you do not increase your likeliness of obtaining the lock by sleeping (OLTP => high frequency, short transactions).

One could add a max counter to avoid endless looping. But then again, you can as well use CTRL-C to interrupt or kill the session.

Cheers