I have mysql table of some records, e.g.:
CREATE TABLE test (
id INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
value varchar NOT NULL
)
now, what I need is to generate unique sequence of 1, 2, ..., N in php script and store to another table... How to achieve this to be thread-safe and not creating doubles or skipping something?
I was wondering if some additional mysql table could be helpful, but I don't know how to create something like "separate autoincrements for each column value" or anything else...
test:
1 ... apples
2 ... oranges
3 ... lemons
some php script (accessed parallely by multiple users at the time):
save_next_fruit($_GET['fruit']);
will create some record in another tables with values like this:
saved_fruit:
ID | FRUIT(FK) | FRUIT_NO
1 1 1
2 1 2
3 2 1
4 3 1
5 3 2
6 1 3
7 3 3
8 2 2
9 1 4
10 2 3
11 1 5
12 2 4
13 1 6
14 3 4
15 3 5
other words, I need to do this (e.g. for fruit 3 (lemons)):
insert into saved_fruit (fruit, fruit_no) values (3, select MAX(fruit_no)+1 from saved_fruit where fruit = 3);
but in thread safe way (I understand that above command is not thread safe in MyISAM MySQL database)
Can you help?
Thanks
MyISAM does support this behavior. Create a two-column primary key, and make the second column auto-increment. It'll start over for each distinct value in the first column.
CREATE TABLE t (i INT, j INT AUTO_INCREMENT, PRIMARY KEY (i,j)) ENGINE=MyISAM;
INSERT INTO t (i) VALUES (1), (1), (2), (2), (1), (3);
SELECT * FROM t;
+---+---+
| i | j |
+---+---+
| 1 | 1 |
| 1 | 2 |
| 1 | 3 |
| 2 | 1 |
| 2 | 2 |
| 3 | 1 |
+---+---+
But if you think about it, this is only thread-safe in a storage engine that does table-level locking for INSERT statements. Because the INSERT has to search other rows in the table to find the max j value per the same i value. If other people are doing INSERTs concurrently, it creates a race condition.
Thus, the dependency on MyISAM, which does table-level locking on INSERT.
See this reference in the manual: http://dev.mysql.com/doc/refman/5.6/en/example-auto-increment.html under the section, MyISAM Notes.
There are a whole lot of good reasons not to use MyISAM. The deciding factor for me is MyISAM's tendency to corrupt data.
Re your comment:
InnoDB does not support the increment-per-group behavior described above. You can make a multi-column primary key, but the error you got is because InnoDB requires that the auto-increment column be the first column in a key of the table (it doesn't strictly have to be the primary key)
Regardless of the position of the auto-increment column in the multi-column key, it only increments when you use it with InnoDB; it does not number entries per distinct value in another column.
To do this with an InnoDB table, you'd have to lock the table explicitly for the duration of the INSERT, to avoid race conditions. You'd do your own SELECT query for the max value in the group you're inserting to. Then insert that value + 1.
Basically, you have to bypass the auto-increment feature and specify values instead of having them automatically generated.
As you using MyISAM you could to lock whole table.
LOCK TABLES `saved_fruit`;
-- Insert query with select.
UNLOCK TABLES;