Question

I convert the DB from Oracle to MySQL.
I’m using both Java & Hibernate.
When I used oracle I had the following method that gave me a brand new and unused sequence value:

protected int getSequenceNextValue() {
    Session session = sessionFactory.getCurrentSession();
    Query query = session.createSQLQuery("select MY_SEQUENCE.NEXTVAL from DUAL");
    return ((BigDecimal) query.uniqueResult()).intValueExact();
}

And I’m trying to refactor this method to work on MySQL DB.
I have a table in MySQL that I use as a sequence (through Hibernate):

create table MY_SEQUENCE(
    next_val int(10) NOT NULL
);

Is there any thread safe way to get a new value from this table and in the same transction to increase it?

For most cases I use the Hibernate Generator to generate a new sequence using this table, but in several cases I need to do it manually.

The best solution for me will be a refactoring of the method above, in such way that threads that querying the table at the same time will not fail, but will wait for each other.

Thanks…

Answer 1

Have a look at the InnoDB table type and FOR UPDATE. An example similar to what you describe is in the MySQL manual here http://dev.mysql.com/doc/refman/5.0/en/innodb-locking-reads.html