Why INSERT IGNORE increments the auto_increment primary key?

Question

I wrote a java program that accesses a MySQL innodb database.

Whenever an INSERT IGNORE statement encounters a duplicate entry the Auto Increment primary key is inc

Answer 1

I think this behaviour is reasonable. The auto-increment should not be relied upon to give sequences that don't have gaps.

For example, a rolled back transaction still consumes IDs:

 INSERT INTO t (normalcol, uniquecol) VALUES 
   ('hello','uni1'),
   ('hello','uni2'),
   ('hello','uni1');

Generates a unique key violation obviously, and inserts no rows into the database (assuming transactional engine here). However, it may consume up to 3 auto-inc values without inserting anything.