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IDS Forum

Re:locking granularity is too big?...

Posted By: ART KAGEL, BLOOMBERG/ 731 LEXIN
Date: Thursday, 20 September 2007, at 1:52 p.m.

In Response To: locking granularity is too big?... (FRANK)

You've got SO MUCH going on here it's hard to know where to start.

First, if you created the table with the default default locking mode, the
locks are taken at the page level and if these are the only few rows in the
table likely the first two are on the same page, so you got locked out. If you
make the tables lock mode 'row' that problem is reduced.

Second, if you begin each session with 'SET LOCK MODE TO WAIT 10;' the engine
will wait ten seconds for transitory locks to be released before returning a
lock timeout error (instead of a lock error). That will prevent instantaneous
locks from causing spurious errors.

Third, be sure to design your apps, especially interactive apps with optimistic
locking (most easily implemented with a timestamp column and INSERT and UPDATE
triggers). This will prevent interactive users from locking rows and index key
nodes for long periods of time preventing other users from working.

Fourth, IDS 11.10 has a new option to COMMITTED READ isolation:

SET ISOLATION COMMITTED READ LAST COMMITTED;
This permits a read-only user to see a consistent view of the data in a table
without encountering any locks on updated rows. The user simply sees the
version of the row that was committed as of the start of the transaction.

Fifth, what makes you think that the INSERT should have been prevented of
should have encoutered a lock? The row has a unique key, guaranteed by the
SERIAL column (BTW you should have a UNIQUE index/constraint on that column),
so there's no reason it cannot be added. The lock encountered by the second
UPDATE is not a row lock or key lock that would prevent the insert. In order
to improve multiuser performance IDS will actually insert rows to multiple
pages at once so this new row added while the page containing the first two
rows is likely being added to another page which itself is not locked. This is
again a misunderstanding because of the effect of page level locking on the two
updates you were attempting.

Art S. Kagel

----- Original Message -----
From: Frank <ids@iiug.org>
To: ids@iiug.org
At: 9/20 13:14:07

HI Folks,

IDS10 UC5

It would be very helpful if IDS could allow the following two
transactions NOT blocking each other in some future Releases.

T1:
begin work;
.......
update abc_test set dt=current
where name='MD'

........

commit work;

before T1 complete, we also started T2, but blocked by T1,

T2:
update abc_test set dt=current
where name='CA';

The interesting thing is, this big locking granularity strategy even does
NOT resolve ghost row problem! For example, it allows the following insert
before T1 commit,

insert into abc_test values(0,'MD',current);

/* attach the test tables
create table "informix".abc_test
(

id serial not null ,

name char(10) not null ,

dt datetime year to second default current year to second,

primary key (id,name) constraint "informix".abc_test_pk
);
*/

The behavior is confused about.

Thanks,
Frank

*******************************************************************************
Forum Note: Use "Reply" to post a response in the discussion forum.

Messages In This Thread

  • locking granularity is too big?...
    FRANK -- Thursday, 20 September 2007, at 1:13 p.m.
    • Re:locking granularity is too big?...
      ART KAGEL, BLOOMBERG/ 731 LEXIN -- Thursday, 20 September 2007, at 1:52 p.m.

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