update my_table set limit_id = 2 where id='176846';
start transaction;
update my_table set limit_id = 1 where id='176846';
update my_table set limit_id = 4 where id='176846'; -- <- this one fails
commit;
select limit_id from my_table where id='176846';
I would like to roll this back automatically - I want the script to output 2, not 1. I have no access to the connection policy in use.
reading here:
http://dev.mysql.com/doc/refman/5.5/en/commit.html
By default, MySQL runs with autocommit mode enabled. This means that
as soon as you execute a statement that updates (modifies) a table,
MySQL stores the update on disk to make it permanent. The change
cannot be rolled back.
try something like
SET autocommit = 0;
start transaction;
(...)
commit;
It depends on why a limit_id value of 4 causes an error, but MySql does not always roll back the entire transaction. See: http://dev.mysql.com/doc/refman/5.7/en/innodb-error-handling.html for more information, but in several cases, MySql will only implicitly rollback the last statement, then continue with the transaction.
Related
I read A Critique of ANSI SQL Isolation Levels, then it talks about cursor lost update as shown below:
But, I don't really understand it so cannot produce it in MySQL.
What is and how to produce cursor lost update in MySQL?
-- initial state
insert into mytable set x = 100;
Trans 1 Trans 2
start transaction; start transaction;
update mytable set x = 75;
update mytable set x = 110;
-- waits because row is locked by trans 1
commit;
-- acquires lock
-- update x = 110 succeeds
commit;
select * from mytable;
-- shows 110
-- where did my value 75 go??
This happens irrespective of cursors. MySQL does not support update where current of cursor as some other implementations do, so the closest you can get is to issue discrete update statements.
Cf. https://dev.mysql.com/doc/refman/8.0/en/cursor-restrictions.html
UPDATE WHERE CURRENT OF and DELETE WHERE CURRENT OF are not implemented, because updatable cursors are not supported.
I am trying to model a transaction database for my databases course. I can't find how to unlock a tuple after using it for update.
I have used commits and assumed that this would release the exclusive lock but it doesn't.
START TRANSACTION;
BEGIN;
SELECT * FROM account WHERE account_num = 3 FOR UPDATE;
UPDATE account SET balance= balance + 100 WHERE account_num = 3;
COMMIT;
What am I supposed to do to make sure this exclusive lock is let go?
START TRANSACTION and BEGIN are synonyms; don't use both.
Unless you are doing something else in the transaction, there is no need for the SELECT, nor for the transaction. UPDATE will lock the row for the duration of the UPDATE.
I want to create a Lost Update with MySQL Workbench. Therefore, I have 2 connections to my database and 2 transactions. I also changed the transaction isolation level to read uncommitted but transaction A uses the current data when the update statement starts. It never uses the data from the first select statement and with select ... for update the transaction b is blocked.
Transaction A (starts first):
Start transaction;
SELECT * FROM table;
Select sleep(10); -- <- Transaction B executes in this 10 seconds
UPDATE table SET Number = Number + 10 WHERE FirstName = "Name1";
COMMIT;
Transaction B:
Start transaction;
UPDATE table SET Number = Number - 5 WHERE FirstName = "Name1";
COMMIT;
Is it possible to create this failure with MySQL Workbench. What´s wrong with my code?
Thanks for your help
The update in A work with data after the sleep is executed. Select before does nothing in the transaction.
I have 5 rows in a table (1 to 5). I want row 2 lock for some update and in the meanwhile if someone tries to update row 4, then he should able to update.
I am trying this with code below, but I feel its placing lock on table level rather than row level.
------ session 1
START TRANSACTION;
SELECT * FROM test WHERE t=1 FOR UPDATE;
UPDATE test SET NAME='irfandd' WHERE t=2;
COMMIT;
----- session 2 (which is being blocked)
START TRANSACTION;
UPDATE test SET NAME='irfandd' WHERE t=4;
COMMIT;
Instead of FOR UPDATE use LOCK IN SHARE MODE. FOR UPDATE prevents other transactions to read the row as well. LOCK IN SHARE MODE allows read, but prevents updating.
Reference: MySQL Manual
------ session 1
START TRANSACTION;
SELECT * FROM test WHERE t=1 LOCK IN SHARE MODE;
UPDATE test SET NAME='irfandd' WHERE t=2;
COMMIT;
----- session 2 (which is not being blocked anymore :) )
START TRANSACTION;
UPDATE test SET NAME='irfandd' WHERE t=4;
COMMIT;
Update:
Realizing that the table has no index on t, I have the following explanation:
First, transaction T1 locks the row 1 in SELECT * FROM test WHERE t=1 FOR UPDATE
Next, transaction T2 tries to execute UPDATE test SET NAME='irfandd' WHERE t=4. To find out which row(s) are affected, it needs to scan all rows, including row 1. But that is locked, so T2 must wait until T1 finishes.
If there is any kind of index, the WHERE t=4 can use the index to decide if row 1 contains t=4 or not, so no need to wait.
Option 1: add an index on test.t so your update can use it.
Option 2: use LOCK IN SHARE MODE, which is intended for putting a read lock only.
Unfortunately this option creates a deadlock. Interestingly, T2 transaction executes (updating row 4), and T1 fails (updating row 2). It seems that T1 read-locks row 4 also, and since T2 modifies it, T1 fails because of the transaction isolation level (REPEATABLE READ by default). The final solution would be playing with Transaction Isolation Levels, using READ UNCOMMITTED or READ COMMITTED transaction levels.
The simplest is Option 1, IMHO, but it's up to your possibilities.
I found below option is more appropriate i generate 40000 numbers from concurrent session on the same time. I didnt found any duplicate number. Without below command i generate 10000 numbers and found 5 duplicate numbers.
START TRANSACTION
SELECT * FROM test WHERE t=1 FOR UPDATE;
UPDATE test SET NAME='irfandd' WHERE t=2;
COMMIT;
In my application I want to take a value from an InnoDB table, and then increment and return it within a single transaction. I want also lock the row that i am going to update in order to prevent another session from changing the value during the transaction. I wrote this query;
SET TRANSACTION ISOLATION LEVEL REPEATABLE READ;
BEGIN TRANSACTION;
SELECT #no:=`value` FROM `counter` where name='booking' FOR UPDATE;
UPDATE `counter` SET `value` = `value` + 1 where `name`='booking';
SELECT #no;
COMMIT;
I want to know if the isolation level is right and is there any need for 'FOR UPDATE' statement. Am i doing it right?
Yes whatever you are doing perfectly fine.
Below lines I am directly quoting from MySQL documentation.
"If you query data and then insert or update related data within the same transaction, the regular SELECT statement does not give enough protection.
..
To implement reading and incrementing the counter, first perform a locking read of the counter using FOR UPDATE, and then increment the counter. For example:
SELECT counter_field FROM child_codes FOR UPDATE;
UPDATE child_codes SET counter_field = counter_field + 1;
A SELECT ... FOR UPDATE reads the latest available data, setting exclusive locks on each row it reads. Thus, it sets the same locks a searched SQL UPDATE would set on the rows.
Reference:
https://dev.mysql.com/doc/refman/5.6/en/innodb-locking-reads.html