Correct me if I'm wrong:

You managed to do the alter tables easier than you'd usually expect. You saw
some user's 4GL programs waiting while you did the ALTER TABLE(s).
That was the good part. The bad part is that one 4GL program instead of
waiting exited with and error and you want to understand why and how to
avoid it....?

If the above is correct, I'm not certain about what may have caused it.
Some things to collect:

1- The query the 4GL was trying to process
2- Is the query previously prepared?

If my assumptions above are not correct, please clarify.
Regards


On Thu, Sep 1, 2011 at 11:45 AM, Habichtsberg, Reinhard <
RHabichtsberg (AT) arz-emmendingen (DOT) de> wrote:



--
Fernando Nunes
Portugal

http://informix-technology.blogspot.com
My email works... but I don't check it frequently...

You got me right.



I now monitored the sql of the running program with onstat -g ses sid -r
1. Under thousands of queries the program produces I found exactely one
(by chance) that ran in modus "not wait". From the code the program
should not change the lock mode. It should be always "wait". But this
could be the reason for the crash.



THX, Reinhard.



From: Fernando Nunes [mailto:domusonline (AT) gmail (DOT) com]
Sent: Thursday, September 01, 2011 1:10 PM
To: Habichtsberg, Reinhard
Cc: informix-list (AT) iiug (DOT) org
Subject: Re: Alter Table non-exclusive



Correct me if I'm wrong:

You managed to do the alter tables easier than you'd usually expect. You
saw some user's 4GL programs waiting while you did the ALTER TABLE(s).
That was the good part. The bad part is that one 4GL program instead of
waiting exited with and error and you want to understand why and how to
avoid it....?

If the above is correct, I'm not certain about what may have caused it.
Some things to collect:

1- The query the 4GL was trying to process
2- Is the query previously prepared?

If my assumptions above are not correct, please clarify.
Regards



On Thu, Sep 1, 2011 at 11:45 AM, Habichtsberg, Reinhard
<RHabichtsberg (AT) arz-emmendingen (DOT) de> wrote:

Hi Fernando,



I now had the situation to do an "alter table" on a highly frequented
table in production system.



I did the following:

- Waited for dialog free time (got up very early in the morning
;-) )

- Killed some dialog sessions from users, which didn't log out

- Monitored some sessions, which which where connected to the
table in question. I found 6 sessions which were active (you saw current
sql statement changing every second). These are workflow batches which
run all the time and one 4gl-program that produces certain reports.

- set IFX_DIRTY_WAIT=300

- Ran the following sql commands:



1 set lock mode to wait;

2 begin work;

3 -- lock table table1 in exclusive mode;

4 grant select on table1 to public;

5 grant select on table2 to public;

6 alter table table1

7 add (column1 char(1))

8 ;

9

10 --lock table table2 in exclusive mode;

11 alter table table2

12 add (column1 char(1))

13 ;

14

15 drop view view1;

16

17 create view view1

18 as select

19 table1.column_x,

.....

470

471 grant select on view1 to public ;

472 grant select on view2 to public;

473

474 commit work;



I didn't believe it before but the "alter table"-statements ran
successfully. The ALTER TABLE of table1 lasted some minutes. I believe
because table1 has 548.675.744 rows fragmented in round robin. I
monitored locks with waiters and found, that the 4gl-program waited for
systable which was locked by the alter table session.



Unfortunately the 4gl-program crashed with an error:



Date: 01.09.2011 Time: 06:32:51

Program error at "ka_st_sl.4gl", line number 2076.

SQL statement error number -243.

Could not position within a table (informix.table1).

SYSTEM error number -106.

ISAM error: non-exclusive access.



The 4gl-programm ran in modus: lockmode wait. I asked the developer and
controlled the code by myself. The time to wait was not restricted.



How can we avoid the crash of a program under these circumstances?



TIA,

Reinhard.



From: Fernando Nunes [mailto:domusonline (AT) gmail (DOT) com]
Sent: Thursday, August 18, 2011 5:22 PM


To: Habichtsberg, Reinhard
Cc: informix-list (AT) iiug (DOT) org
Subject: Re: Alter Table non-exclusive



Don't get me wrong. I think every Informix customer has "dirty readers".
And currently most of the OLTP systems are also used for decision
support queries (or at least for ETL).
In that case, and someone else already explained, IFX_DIRTY_WAIT will
certainly help. But, back to basics, you may have a problem if you have
queries on the affected tables that last for days. I'm saying this
because all this talk comes down to one thing: identify and stop the
sessions that prevent the access.

For a complete personal overview about this topic check:

http://informix-technology.blogspot....sive-is-not-re
ally-exclusive.html

Regards.

On Thu, Aug 18, 2011 at 3:14 PM, Habichtsberg, Reinhard
<RHabichtsberg (AT) arz-emmendingen (DOT) de> wrote:

Fernando,



we have DIRTY READERS on the tables, yes, we have a "mixed system" with
OLTP and decision support with queries which can last days.



The IFX_DIRTY_WAIT variable may help for short dirty reads. I have to
think about it and will do some tests. Meanwhile I thank you for your
help.



Reinhard.



From: Fernando Nunes [mailto:domusonline (AT) gmail (DOT) com]
Sent: Thursday, August 18, 2011 2:02 PM


To: Habichtsberg, Reinhard
Cc: informix-list (AT) iiug (DOT) org
Subject: Re: Alter Table non-exclusive



We're having a semantic problem. What you say is true. If an exclusive
lock is needed, than you can't get it while the table is in use. But
that's a general question regarding locks. That's why we have lock
mode...
If the sessions leave open cursors on the tables for a long period than
you'll have issues.
Again, having a downtime in production is a "scary way" of putting it.
Having to hold or interrupt a few sessions for a few seconds is much
more understandable.
As someone else already mentioned, if you're having DIRTY READERS on the
tables the situation is much more complex.
And as I reference in my blog article you can use a small trick to
prevent new sessions of getting into the way: Open a transaction, grant
some privilege to the table and then run the alter inside that
transaction. The grant will place a lock on the table's systable record
and this prevents other sessions (assuming they're in committed read and
lock mode wait) to read the table structure. So the sessions will wait
on systables and not on the table itself.
This is useless for dirty readers, but for those you'll have the
IFX_DIRTY_WAIT variable.

Meanwhile, while searching interanlly for other things I found a feature
request for this. But the issue is complex and not easy to solve...
meaning there's no compromise that it will be implemented. It just means
that formally IBM knows about this concern.

Regards.

On Thu, Aug 18, 2011 at 6:34 AM, Habichtsberg, Reinhard
<RHabichtsberg (AT) arz-emmendingen (DOT) de> wrote:

If an exclusive lock is required it is impossible to do the job while
the table is in use. And some tables in our environment are in use of
one or more session permanently. So it doesn't matter how long the alter
job lasts. It cannot be accomplished without a downtime of production.



That is what we (better: my boss) wish(es) to improve.



@Fernando: To find the sessions that use a table I found a tool in the
IIUG software corner long ago:

1 #!/usr/bin/ksh

2 # who-access.sh - Find out who is accessing a specified table.

3 #

4 # Author: Jacob Salomon

5 # Date: 07/15/1999

6 # Credits: Original idea from a script by:

7 # Rick Bernstein ...



It's really very usefull and reliable.



Reinhard.



From: Art Kagel [mailto:art.kagel (AT) gmail (DOT) com]
Sent: Wednesday, August 17, 2011 1:26 PM
To: Habichtsberg, Reinhard
Cc: informix-list (AT) iiug (DOT) org
Subject: Re: Alter Table non-exclusive



Some things already only require an exclusive lock momentarily. Most
alters that add or drop a column or change it's type, adding or dropping
an index, and others are accomplished in-place or online and do not lock
that table for long. What are you trying to do that's causing you
trouble?

Art

Art S. Kagel
Advanced DataTools (www.advancedatatools.com)
Blog: http://informix-myview.blogspot.com/

Disclaimer: Please keep in mind that my own opinions are my own opinions
and do not reflect on my employer, Advanced DataTools, the IIUG, nor any
other organization with which I am associated either explicitly,
implicitly, or by inference. Neither do those opinions reflect those of
other individuals affiliated with any entity with which I am affiliated
nor those of the entities themselves.

On Wed, Aug 17, 2011 at 6:17 AM, Habichtsberg, Reinhard
<RHabichtsberg (AT) arz-emmendingen (DOT) de> wrote:

Hi all



We have 24 x 7 production with our Informix databases. We all heard of
Informix Server that ran a year or longer without interruption
<http://dict.leo.org/ende?lp=ende&p=C...uption&tres t
r=0x8001> .



What we wish is the ability to do "Alter tables" and other
administrative tasks that needs "exclusive locks" without this need.
Each change on the table structures which come continuous from our
software developement causes trouble with our production: Workflows has
to be stopped, dialog programs can't run.



Is there any chance do these tasks without exclusive locks, perhaps in
futur? Or, is there any other way to do these tasks without a production
break?



TIA, Reinhard.


_______________________________________________
Informix-list mailing list
Informix-list (AT) iiug (DOT) org
http://www.iiug.org/mailman/listinfo/informix-list




_______________________________________________
Informix-list mailing list
Informix-list (AT) iiug (DOT) org
http://www.iiug.org/mailman/listinfo/informix-list




--
Fernando Nunes
Portugal

http://informix-technology.blogspot.com
My email works... but I don't check it frequently...


_______________________________________________
Informix-list mailing list
Informix-list (AT) iiug (DOT) org
http://www.iiug.org/mailman/listinfo/informix-list




--
Fernando Nunes
Portugal

http://informix-technology.blogspot.com
My email works... but I don't check it frequently...




--
Fernando Nunes
Portugal

http://informix-technology.blogspot.com
My email works... but I don't check it frequently...

Hmmmm.....
The LOCK MODE is a session property. Different parts of the code (or even
inside stored procedures), this can be changed and not put back.
(By the way, I would love to see some extension to run each query in a
particular isolation level and lock mode. I know ANSI doesn't allow it to
change - SET TRANSACTION... - but our extension SET ISOLATION... can be
used. So imagine a "SELECT .... WITH COMMITTED READ LOCK MODE WAIT 5" )

Besides what you found I don't see any explanation to what happened to
you... Maybe others have some clue (although this can explain it...)?

Regarding the change of isolation level and lock mode inside "functions" or
procedures, this is also an interesting thing. The fact is that sometimes
you want/need to change it and it's not easy to find what is the current
one, so that you can reposition it once you're done.
Sometime ago I dig into this and it's possible to have a procedure that
implements it... I thought I had an article about this but I don't seem to
find it...

Regards


On Thu, Sep 1, 2011 at 3:20 PM, Habichtsberg, Reinhard <
RHabichtsberg (AT) arz-emmendingen (DOT) de> wrote:



--
Fernando Nunes
Portugal

http://informix-technology.blogspot.com
My email works... but I don't check it frequently...

You should not get an ISAM error -106 with LOCK MODE WAIT set with no
timeout value!

Art

Art S. Kagel
Advanced DataTools (www.advancedatatools.com)
Blog: http://informix-myview.blogspot.com/

Disclaimer: Please keep in mind that my own opinions are my own opinions and
do not reflect on my employer, Advanced DataTools, the IIUG, nor any other
organization with which I am associated either explicitly, implicitly, or by
inference. Neither do those opinions reflect those of other individuals
affiliated with any entity with which I am affiliated nor those of the
entities themselves.



On Thu, Sep 1, 2011 at 6:45 AM, Habichtsberg, Reinhard <
RHabichtsberg (AT) arz-emmendingen (DOT) de> wrote: