增量计数器mysql

Question

My question is pretty simple but answer might be tricky.

I'm in PHP and I want to manage manually a unique ID for my objects. What is tricky is to manage atomicity. I dont want that 2 elements get the same ID.

"Elements" are grouped in "Groups". In each group I want elements ID starting from 1 and grow incrementally for each insert in that group.

My first solution is to have a "lastID" column in the table "Groups" :

CREATE TABLE groups ( id INT AUTO_INCREMENT, lastId INT )
CREATE TABLE elements ( myId INT, multiple values ...)

In order to avoid many elements with the same ID, I have to update lastId and select it in an atomic SQL Query.

After that, one retrieved, I have a unique ID that can't be picked again and I can insert my element.

My question is how to solve the bold part ? My database is MySQL with MyISAM engine so there is no transaction support.

UPDATE groups 
SET lastId = lastId + 1 
WHERE id = 42

SELECT lastId 
FROM groups
WHERE id = 42

Is there something more atomic than these 2 requests ?

Thanks

Answer 1

UPDATE groups SET lastId = last_insert_id(lastId + 1)

and then you can get your new id with

SELECT last_insert_id()

Using last_insert_id with a parameter will store the value and return it when you call it later.
This method of generating autonumbers works best with MyISAM tables having only a few rows (MyISAM always locks the entire table). It also has the benefit of not locking the table for the duration of the transaction (which will happen if it is an InnoDB table).

This is from the MySQL manual :

If expr is given as an argument to LAST_INSERT_ID(), the value of the argument is returned by the function and is remembered as the next value to be returned by LAST_INSERT_ID(). This can be used to simulate sequences:

Create a table to hold the sequence counter and initialize it:

CREATE TABLE sequence (id INT NOT NULL); 
INSERT INTO sequence VALUES (0); 

Use the table to generate sequence numbers like this:

UPDATE sequence SET id=LAST_INSERT_ID(id+1);
SELECT LAST_INSERT_ID(); 

The UPDATE statement increments the sequence counter and causes the next call to LAST_INSERT_ID() to return the updated value. The SELECT statement retrieves that value. The mysql_insert_id() C API function can also be used to get the value. See Section 21.8.3.37, “mysql_insert_id()”.

You can generate sequences without calling LAST_INSERT_ID(), but the utility of using the function this way is that the ID value is maintained in the server as the last automatically generated value. It is multi-user safe because multiple clients can issue the UPDATE statement and get their own sequence value with the SELECT statement (or mysql_insert_id()), without affecting or being affected by other clients that generate their own sequence values.

Answer 2

One option is for you to use the nifty MyISAM feature that let's auto_increment values be incremented for each group.

CREATE UNIQUE INDEX elements_ix1 ON elements (groupId, myID)

myID INT NOT NULL AUTO_INCREMENT

That's more "atomic" than anything that involves updating a separate table. Note that this only works for MyISAM, not InnoDB.

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excerpt from http://dev.mysql.com/doc/refman/5.1/en/example-auto-increment.html

MyISAM Notes

For MyISAM tables, you can specify AUTO_INCREMENT on a secondary column in a multiple-column index. In this case, the generated value for the AUTO_INCREMENT column is calculated as MAX(auto_increment_column) + 1 WHERE prefix=given-prefix. This is useful when you want to put data into ordered groups.

Answer 3

I would assume your MySQL installation also has InnoDB engine which does support transactions. You just need to change the engine type of you tables.