Hi Baron,
Thank you for your answer.
The problem is that multiple users can read and update the same field
simultaneously (worse case) which could lead to unpredictable problems.
According to the MySql online documentation
(http://dev.mysql.com/doc/refman/5.0/en/innodb-locking-reads.html) this is
the way to solve this problem.
What I really want is the following:
When person A is reading and updating a field value, person B should not be
able to do this simultaneously. Person B has to wait till the Person A has
finished his work.
Are there other more robust solutions to solve such a problem?
Thank you in advance.
Regards,
Johan Machielse
----- Original Message -----
From: "Baron Schwartz" <ba...@xaprb.com>
To: "Johan Machielse" <johan.machie...@kpnplanet.nl>
Cc: <mysql@lists.mysql.com>
Sent: Friday, January 15, 2010 3:14 AM
Subject: Re: When using "FOR UPDATE" whole the table seems to lock instead
of selected row
Johan,
I don't see a valid need for using FOR UPDATE here. In fact, FOR
UPDATE is the cause of many grievances, and I would advise you to
avoid it by any means possible. Among other things, it will cause
serious performance problems when your server gets busy. And as you
can see, it's hard to figure out why it behaves as it does. Simply
issue the UPDATE statement without a SELECT first.
- Baron
On Thu, Jan 14, 2010 at 4:08 AM, Johan Machielse
<johan.machie...@kpnplanet.nl> wrote:
Hi,
I have created a query to read and update a stock item by using the FOR
UPDATE statement. According to the MySql documention only the rows that
are selected using the FOR UPDATE should be locked for other sessions, but
somehow whole the table is locked. This post gives some general
information and then shows 3 scenarios. The first 2 scenarios work as
expected, but the last one fails.
Two questions:
a.. Does anyone has an idea why scenario 3 does not work as expected and
what is the solution to make it work?
b.. Is this the preferred way to update a stock table? If not, could you
provide me an example how it should be done?
Here some clarification about the problem:
Version MySQL
5.0.45-community-nt
Stock table
DROP TABLE IF EXISTS `mydatabase`.`stock`;
CREATE TABLE `mydatabase`.`stock` (
`ID` bigint(20) unsigned NOT NULL auto_increment,
`ProductID` bigint(20) unsigned NOT NULL,
`SizeID` bigint(20) unsigned NOT NULL,
`Quantity` int(11) NOT NULL,
PRIMARY KEY USING BTREE (`ID`)
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=latin1;
MySQL example
This query updates the stock of one product of a particular size by
decrementing it's quantity by 1.
START TRANSACTION;
SET AUTOCOMMIT=0;
SELECT Stock.Quantity FROM Stock WHERE Stock.ProductID = 1 AND
Stock.SizeID = 2 FOR UPDATE;
UPDATE Stock SET Stock.Quantity = Stock.Quantity - 1 WHERE Stock.ProductID
= 1 AND Stock.SizeID = 2;
COMMIT;
Now let me show you three scenarios wherein you can see that using the FOR
UPDATE statement is not working as it should.
Scenario 1 (works as expected)
I opened two sessions of MySql Query Browser and each executing the same
MySql query. So, both looking at the same row (same selection criteria).
START TRANSACTION;
SET AUTOCOMMIT=0;
SELECT Stock.Quantity FROM Stock WHERE Stock.ProductID = 1 AND
Stock.SizeID = 2 FOR UPDATE;
UPDATE Stock SET Stock.Quantity = Stock.Quantity - 1 WHERE Stock.ProductID
= 1 AND Stock.SizeID = 2;
COMMIT;
1) Session 1: START TRANSACTION;
2) Session 1: SET AUTOCOMMIT=0;
3) Session 1: SELECT Stock.Quantity FROM Stock WHERE Stock.ProductID = 1
AND Stock.SizeID = 2 FOR UPDATE;
(returns Stock.Quantity as expected)
4) Session 2: START TRANSACTION;
5) Session 2: SET AUTOCOMMIT=0;
6) Session 2: SELECT Stock.Quantity FROM Stock WHERE Stock.ProductID = 1
AND Stock.SizeID = 2 FOR UPDATE;
(blocks as exepected)
7) Session 1: UPDATE Stock SET Stock.Quantity = Stock.Quantity - 1 WHERE
Stock.ProductID = 1 AND Stock.SizeID = 2;
8) Session 1: COMMIT;
(the blocking step 6 is now executed and returns the updated
Stock.Quantity as exepected)
9) Session 2: UPDATE Stock SET Stock.Quantity = Stock.Quantity - 1 WHERE
Stock.ProductID = 1 AND Stock.SizeID = 2;
10) Session 2: COMMIT;
Scenario 2 (works as expected)
I opened two sessions of MySql Query Browser and each executing the same
MySql query. Only the selection criteria are different;
both are using another value for the primary key "ID". When I use
different values for the primary key "ID" I have the same results
as in scenario 1.
Session 1 query
START TRANSACTION;
SET AUTOCOMMIT=0;
SELECT Stock.Quantity FROM Stock WHERE Stock.ID = 1 FOR UPDATE;
UPDATE Stock SET Stock.Quantity = Stock.Quantity - 1 WHERE Stock.ID = 1;
COMMIT;
Session 2 query
START TRANSACTION;
SET AUTOCOMMIT=0;
SELECT Stock.Quantity FROM Stock WHERE Stock.ID = 2 FOR UPDATE;
UPDATE Stock SET Stock.Quantity = Stock.Quantity - 1 WHERE Stock.ID = 2;
COMMIT;
1) Session 1: START TRANSACTION;
2) Session 1: SET AUTOCOMMIT=0;
3) Session 1: SELECT Stock.Quantity FROM Stock WHERE Stock.ID = 1 FOR
UPDATE;
(returns Stock.Quantity as expected)
4) Session 2: START TRANSACTION;
5) Session 2: SET AUTOCOMMIT=0;
6) Session 2: SELECT Stock.Quantity FROM Stock WHERE Stock.ID = 2 FOR
UPDATE;
(no blocking as exepected, because it's another row; returns
Stock.Quantity as expected)
7) Session 1: UPDATE Stock SET Stock.Quantity = Stock.Quantity - 1 WHERE
Stock.ID = 1;
8) Session 1: COMMIT;
9) Session 2: UPDATE Stock SET Stock.Quantity = Stock.Quantity - 1 WHERE
Stock.ID = 2;
10) Session 2: COMMIT;
Scenario 3 (does not work as expected)
I opened two sessions of MySql Query Browser and each executing the same
MySql query. Only the selection criteria are different;
both are using another value for the non-primary key "SizeID". Making
SizeID a primary key does not influence the results of this
scenario. So, the sessions work in different rows, but session 2 is
blocked! It looks like the whole table is locked instead of only one row?
Session 1 query
START TRANSACTION;
SET AUTOCOMMIT=0;
SELECT Stock.Quantity FROM Stock WHERE Stock.ProductID = 1 AND
Stock.SizeID = 1 FOR UPDATE;
UPDATE Stock SET Stock.Quantity = Stock.Quantity - 1 WHERE Stock.ProductID
= 1 AND Stock.SizeID = 1;
COMMIT;
Session 2 query
START TRANSACTION;
SET AUTOCOMMIT=0;
SELECT Stock.Quantity FROM Stock WHERE Stock.ProductID = 1 AND
Stock.SizeID = 2 FOR UPDATE;
UPDATE Stock SET Stock.Quantity = Stock.Quantity - 1 WHERE Stock.ProductID
= 1 AND Stock.SizeID = 2;
COMMIT;
1) Session 1: START TRANSACTION;
2) Session 1: SET AUTOCOMMIT=0;
3) Session 1: SELECT Stock.Quantity FROM Stock WHERE Stock.ProductID = 1
AND Stock.SizeID = 1 FOR UPDATE;
(returns Stock.Quantity as expected)
4) Session 2: START TRANSACTION;
5) Session 2: SET AUTOCOMMIT=0;
6) Session 2: SELECT Stock.Quantity FROM Stock WHERE Stock.ProductID = 1
AND Stock.SizeID = 2 FOR UPDATE;
(blocks, but this is not exepected and is unwanted behaviour, because
session 2 is using another row)
7) Session 1: UPDATE Stock SET Stock.Quantity = Stock.Quantity - 1 WHERE
Stock.ProductID = 1 AND Stock.SizeID = 1;
8) Session 1: COMMIT;
(the blocking step 6 is now executed and returns the Stock.Quantity as
exepected)
9) Session 2: UPDATE Stock SET Stock.Quantity = Stock.Quantity - 1 WHERE
Stock.ProductID = 1 AND Stock.SizeID = 2;
10) Session 2: COMMIT;
Regards,
Johan Machielse
Machielse Software
http://www.machielsesoftware.nl
--
Baron Schwartz
Percona Inc: Services and Support for MySQL
http://www.percona.com/
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