Strange behaviour from MYSQL 5 (Database Isolation) - mysql

Question

I opened two command windows to work with my Database (MySQL5).
Below is table structure I'm working with (It should be noted that I've turned off the auto commit by executing set autocommit=0;):
Table Structure:
CREATE TABLE `ajax`.`zipcodes` (
`ZIPCODE` varchar(5) NOT NULL,
`CITY` varchar(50) DEFAULT NULL,
`STATE` varchar(2) DEFAULT NULL,
PRIMARY KEY (`ZIPCODE`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1;
Below is sequence of activities:
Step 1:
In command window 1, I executed below commands and you could also see the output:
mysql> insert into ajax.zipcodes values(5, 'Wil', 'AK');
Query OK, 1 row affected (0.00 sec)
Step 2
In second command window, I fired below command and it hangs (it seems waiting for commit command to be issues from previous window)
mysql> update ajax.zipcodes set city='Dublin' where zipcode=5;
Step 3
I went to Command window#1, and executed commit; you could see the output below:
mysql> commit;
Query OK, 0 rows affected (0.00 sec)
At the same time, i could see that second window that was earlier hanging, also executed the command and printed below output:
mysql> update ajax.zipcodes set city='Dublin' where zipcode=5;
Query OK, 1 row affected (3.63 sec)
Rows matched: 1 Changed: 1 Warnings: 0
Step 4
Now I issues commit in my second window to ensure that all the changes gets commited properly even the second session:
mysql> commit;
Query OK, 0 rows affected (0.00 sec)
Step 5
Now since commit has been issued from both windows I thought that everything is all right and the two session also must be in sync, so I went to 1st command window and issues below command:
mysql> select * from zipcodes where zipcode=5;
+---------+------+-------+
| ZIPCODE | CITY | STATE |
+---------+------+-------+
| 5 | Wil | AK |
+---------+------+-------+
1 row in set (0.00 sec)
I was surprised because I was expecting City value to be 'Dublin' because changes from second command window (i.e. update) has been commited in Step 4, but i'm still getting Wil in City column.
What am I doing wrong here?

Answer 1

This is to do with isolation levels. If you raise your isolation level to SERIALIZABLE (the default in MySQL is REPEATABLE READS) you won't get "phantom reads".
Isolation levels and phantom reads are described on the Wikipedia page for database transaction isolation.
If I run this through as you did, but with the higher isolation level, I get the result you were expecting.
Session 1
mysql> set autocommit=0;
Query OK, 0 rows affected (0.00 sec)
mysql> SET SESSION TRANSACTION ISOLATION LEVEL SERIALIZABLE;
Query OK, 0 rows affected (0.00 sec)
mysql> CREATE TABLE `ajax`.`zipcodes` (
-> `ZIPCODE` varchar(5) NOT NULL,
-> `CITY` varchar(50) DEFAULT NULL,
-> `STATE` varchar(2) DEFAULT NULL,
-> PRIMARY KEY (`ZIPCODE`)
-> ) ENGINE=InnoDB DEFAULT CHARSET=latin1;
Query OK, 0 rows affected (0.07 sec)
mysql> insert into ajax.zipcodes values(5, 'Wil', 'AK');
Query OK, 1 row affected (0.00 sec)
Session 2
mysql> set autocommit=0;
Query OK, 0 rows affected (0.00 sec)
mysql> SET SESSION TRANSACTION ISOLATION LEVEL SERIALIZABLE;
Query OK, 0 rows affected (0.00 sec)
mysql> update ajax.zipcodes set city='Dublin' where zipcode=5;
Session 1
mysql> commit;
Query OK, 0 rows affected (0.04 sec)
Session 2
/* continued from previous (was frozen) */
Query OK, 1 row affected (7.54 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> select * from zipcodes;
+---------+--------+-------+
| ZIPCODE | CITY | STATE |
+---------+--------+-------+
| 5 | Dublin | AK |
+---------+--------+-------+
1 row in set (0.00 sec)
mysql> commit;
Query OK, 0 rows affected (0.04 sec)
mysql> select * from zipcodes;
+---------+--------+-------+
| ZIPCODE | CITY | STATE |
+---------+--------+-------+
| 5 | Dublin | AK |
+---------+--------+-------+
1 row in set (0.00 sec)
Session 1
mysql> select * from zipcodes;
+---------+--------+-------+
| ZIPCODE | CITY | STATE |
+---------+--------+-------+
| 5 | Dublin | AK |
+---------+--------+-------+
1 row in set (0.00 sec)
NB: This doesn't necessarily mean that you should always be using SERIALIZABLE - there are trade-offs. Most notable is that the database will acquire a range lock when executing a SELECT and you'll get more locking-based conflicts.
Update - Explicitly Handling Transactions
Since we have autocommit=0; set in these scripts, we really should handle the transactions explicitly, rather than expecting a START TRANSACTION - although in most cases, the database behaves as you'd expect if you'd executed START TRANSACTION.
However, run the original example while explicitly starting and ending all transactions (including those that are just SELECT, and you get a different result:
Session 1
mysql> set autocommit=0;
Query OK, 0 rows affected (0.00 sec)
mysql> SET SESSION TRANSACTION ISOLATION LEVEL REPEATABLE READ;
Query OK, 0 rows affected (0.00 sec)
mysql> CREATE TABLE `ajax`.`zipcodes` (
-> `ZIPCODE` varchar(5) NOT NULL,
-> `CITY` varchar(50) DEFAULT NULL,
-> `STATE` varchar(2) DEFAULT NULL,
-> PRIMARY KEY (`ZIPCODE`)
-> ) ENGINE=InnoDB DEFAULT CHARSET=latin1;
Query OK, 0 rows affected (0.07 sec)
mysql> START TRANSACTION;
Query OK, 0 rows affected (0.00 sec)
mysql> insert into ajax.zipcodes values(5, 'Wil', 'AK');
Query OK, 1 row affected (0.00 sec)
Session 2
mysql> set autocommit=0;
Query OK, 0 rows affected (0.00 sec)
mysql> SET SESSION TRANSACTION ISOLATION LEVEL REPEATABLE READ;
Query OK, 0 rows affected (0.00 sec)
mysql> START TRANSACTION;
Query OK, 0 rows affected (0.00 sec)
mysql> update ajax.zipcodes set city='Dublin' where zipcode=5;
Session 1
mysql> commit;
Query OK, 0 rows affected (0.04 sec)
Session 2
/* continued from previous (was frozen) */
Query OK, 1 row affected (8.32 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> select * from zipcodes;
+---------+--------+-------+
| ZIPCODE | CITY | STATE |
+---------+--------+-------+
| 5 | Dublin | AK |
+---------+--------+-------+
1 row in set (0.00 sec)
mysql> commit;
Query OK, 0 rows affected (0.04 sec)
mysql> START TRANSACTION;
Query OK, 0 rows affected (0.00 sec)
mysql> select * from zipcodes;
+---------+--------+-------+
| ZIPCODE | CITY | STATE |
+---------+--------+-------+
| 5 | Dublin | AK |
+---------+--------+-------+
1 row in set (0.00 sec)
mysql> commit;
Query OK, 0 rows affected (0.04 sec)
Session 1
mysql> START TRANSACTION;
Query OK, 0 rows affected (0.00 sec)
mysql> select * from zipcodes;
+---------+--------+-------+
| ZIPCODE | CITY | STATE |
+---------+--------+-------+
| 5 | Dublin | AK |
+---------+--------+-------+
1 row in set (0.00 sec)