Is there a practical way of migrating from identity columns to hilo keys?

Question

I work with a database that depends heavily on identity columns. However as we have now moved all applications over to NHibernate I wanted to look into using HiLo as seems to be

Answer 1

Here is a sample from a recent migration from the increment generator to the MultipleHiLoPerTableGenerator (e.g. a single table is used to store high values for every Entities).

My application is using Hibernate 3 + mapping files (.hbm.xml). My database is MySQL (innoDB + auto increment pk).

Step 1 : replace your generator settings in your .hbm files. Replace :

By

    hilo_values
    sequence_name
    sequence_next_hi_value
    1000

Step 2 : create a new table to store the high values

CREATE TABLE IF NOT EXISTS `hilo_values` (
  `sequence_name` varchar(255) NOT NULL,
  `sequence_next_hi_value` int(11) NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

Step 3 : populate the initial high values according to existing data using the following piece of SQL. I assume here that the same max_lo value is used for every table.

INSERT INTO hilo_values SELECT TABLE_NAME,  ((AUTO_INCREMENT DIV (1000 + 1)) + 1) FROM information_schema.tables WHERE table_schema = 'yourdbname'