I wrote a bunch of delete statements and wrapped them in a transaction:
start transaction;
delete a...
delete b...
delete c...
rollback;
The idea is I would want the deletes to occur inside a transaction which would rollback upon completion. If a step failed along the way, I would want the successful steps to be rolled back as well.
To my chagrin delete a worked, removing a few thousand rows, delete b failed, but when I reran the statements all the records from a appeared to be gone.
Is this because the transaction is still open? I tried doing:
set session transaction isolation level read committed;
select a.*
and got back zero rows so I think that is not the case. Was the successful a delete committed? And if so, how do I prevent that from happening until I can guarantee a full working query?
MySQL Workbench enables auto commit by default. In the SQL editor there is a toolbar button that can be used to toggle auto commit at will:
Somebody turned me on to, what I think, is a better way to do this:
begin;
<sql transactions>
commit;
Try setting the autocommit flag to false:
set ##autocommit = false;
start transaction;
delete a...
delete b...
delete c...
rollback;
Related
I was wondering what happens when several users try to UPDATE a MySQL database at the same time. Went online and learnt about Transactions.
Do I need to define the transactions statements.(START TRANSACTION,... COMMIT/ROLLBACK) for every one of my database UPDATE queries? Is there an automated way to achieve this?
By default , MySQL has autocommit enabled, which can be toggled off using
set session autocommit=off;
For transaction based queries like banking, manual use of TRANSACTION is executed, which ignores the autocommit setting. e.g:
START TRANSACTION;
update bank_card set balance=balance-100;
update credit_card set balance=balance+100;
ROLLBACK; -- execute rollback to cancel the transaction if necessary
COMMIT;
Should something went wrong in any of the UPDATE statements,by executing ROLLBACK; the transaction is aborted and no UPDATE is performed.
When several users try to UPDATE the same table, MySQL will place a LOCK on related rows which prevents simultaneous writing. The first UPDATE query will release the lock by either committing (successful) or rolling back (usually something went wrong) before the second update query can acquire the lock and do the UPDATE.
Let's say I am trying to execute the following UPDATE statement in mysql (Innodb):
UPDATE main SET name = "Ralph" WHERE rowid=19283
Is there a way before doing this statement to see if there is a row/table-level lock on rowid=19283 before running this update? Or is the application strategy of dealing with deadlocks to catch the exception and then deal with them after the fact? I find that once a deadlock is reached, it's often impossible to update that row without some very loop-y logic, and so I'm seeing if the deadlock can be detected before the potential UPDATE/INSERT statement
A common pattern is
BEGIN;
SELECT ... FOR UPDATE; -- grab a lock on the row
... do some other processing, then eventually:
UPDATE ... -- change the row (or maybe skip this in some cases)
COMMIT;
This allows multiple connections to gracefully change that row, but without stepping on each other.
No, this does not eliminate deadlocks. It may turn a deadlock into a "lock wait", which is fine.
And it is not quite a "dry run". It moves the lock from the UPDATE back to the SELECT. If there are other things going on in this, and the competing, transaction, there could be a deadlock.
If you have 2 connections doing that transaction at "exactly" the same time, one will wait until the other finishes. (No deadlock.)
So this is from the docs:
To disable autocommit mode implicitly for a single series of
statements, use the START TRANSACTION statement:
START TRANSACTION;
SELECT #A:=SUM(salary) FROM table1 WHERE type=1;
UPDATE table2 SET summary=#A WHERE type=1;
COMMIT;
So from my understanding, unlike using BEGIN; command which shouldn't set autocommit to 0, START TRANSACTION should set it to 0.
Now if I do this (after I start a transaction):
select ##autocommit;
I get the value of 1.
Why autocommit is still enabled even if I use START TRANSACTION command? I thought that the autocommit variable is local to a single session. Or maybe even if it says 1, it is actually set to 0 within a transaction but just can't get that info by running a SELECT ##autocommit; query?
https://dev.mysql.com/doc/refman/5.7/en/commit.html says:
With START TRANSACTION, autocommit remains disabled until you end the transaction with COMMIT or ROLLBACK. The autocommit mode then reverts to its previous state.
The value of the autocommit variable is where MySQL stores the "previous state" so that it can revert to it after your transaction completes.
You can experimentally confirm that this behavior is followed. Make a change in a transaction, and then roll back the transaction.
CREATE TABLE test.MyTable (mycolumn TEXT);
START TRANSACTION;
INSERT INTO test.MyTable (mycolumn) VALUES ('Thing 1');
ROLLBACK;
SELECT * FROM test.MyTable; -- finds the row is gone
Notice that the change you did is rolled back. If autocommit had been in effect, then rollback would never work, because each statement would commit as soon as it is executed.
I suggest that ##autocommit is irrelevant. The engine knows that it is in a transaction (START or BEGIN), so it ignores the setting of autocommit. Instead it hangs onto changes until COMMIT (or ROLLBACK).
Or do you have some reason to believe that the value of autocommit is relevant? (Aside from SELECT ##autocommit.)
Consider a transaction T1,
Start transaction;
Update emp set emp_id=1 where emp_id=3;
commit;
The engine i am using is INNODB engine.
Before commit operation of the above shown transaction, I had accessed the table again it is showing the previous committed values. If the Row Level locking is placed on the table, it might have shown the error (you cannot access while some transaction is in the middle). Is there any wrong in my understanding.? Can any one help me on this?
Anything that is done as a part of a transaction is available to the same transaction even before the transaction is committed. The changes are not available in other transactions.
To test this, you need to update in one transaction and then from another terminal start a new transaction and try to access. The second transaction will be able to read the data but if you try to update the update will block and wait for the first transaction to be committed.
If you want the second select to wait and return the updated data you should use select for update.
I have a user table, and I have 5 records.
I deleted two records, then executed the rollback command, it executed successfully.
But that deleted two records not recovered.
The user table engine is InnoDB.
You should be able to rollback your transaction as the table engine is InnoDB.
Anyways here is the correct way to do transactions,
SET autocommit=0;
START TRANSACTION;
Your Query here.
ROLLBACK;
and make sure that you are not using COMMIT after the Query which you need to rollback. Refer
Table Engines and Transaction. And When a DB connection is created, it is in auto-commit mode by default. This means that each individual SQL statement is treated as a transaction and is automatically committed right after it is executed. So if you need to do transactions yourself, you must turn off the autocommit mode by AUTOCOMMIT = 0. Refer this link for more info.
By default, MySQL starts the session
for each new connection with
autocommit enabled,
You could set autocommit mode disable before running your query
SET autocommit=0;
ref:
Link 1
Link 2
Make sure you already do command
start transaction;
before the query delete.
SET autocommit=0;
BEGIN;
.
.
.
ROLLBACK;
START TRANSACTION, COMMIT, and ROLLBACK Syntax
I don't know if you were locking tables, but I ran into the same problem where it seemed like rollback wasn't working.
Within my transaction I called "LOCK TABLES...", which
implicitly commits any active transaction before attempting to lock
the tables
(http://dev.mysql.com/doc/refman/5.0/en/lock-tables-and-transactions.html)
I ended up locking outside of (before) the transaction, as that was the first thing I was doing in the transaction anyway. Alternatively, you can do as the docs suggest:
SET autocommit=0;
LOCK TABLES t1 WRITE, t2 READ, ...;
... do something with tables t1 and t2 here ...
COMMIT;
UNLOCK TABLES;
I have the same problem, but i has checked the innodb support
First:
Verify if the Engine INNODB is Available: with this:
mysql> show engines;
if INNODB is disabled: then
Second:
go to the file "my.ini", in my case is in C:\AppServ\mysql
in this file removes the "#" of the line
#default-storage-engine=INNODB -> default-storage-engine=INNODB
and the line skip-innodb add "#"
skip-innodb -> #skip-innodb
Third:
save and restart mysql service, and the problem was solved.
I think there is one important thing to mention: to re-enable autocommit after transaction is done.
SET autocommit = 0;
START TRANSACTION;
INSERT INTO ..
UPDATE <table> ..
COMMIT;
SET autocommit = 1;
Otherwise anything you do after disabling autocommit even without transaction will require you to explicitly commit.
If you're using MySQL Workbench you can turn off Auto-commit transactions in 'Query' tab.
And don't forget to use "COMMIT;" after the Query which you need to rollback.
Even for me rollback was not working for InnoDB engine.
But adding begin; statement after start transaction; made the fix.
This worked for me
SET autocommit = 0;
start transaction;
begin;
-- DML query goes here
rollback;
SET autocommit = 1;
But
This doesn't worked for me
SET autocommit = 0;
start transaction;
-- DML query goes here
rollback;
SET autocommit = 1;
Really i don't know the reason, If anyone knows please comment here.