春季数据-JPA，缺少ID

Question

We are using a spring data project, we have a mysql database. Lately we have noticed the following on the USER table:

id  |   name |  pass | email
=====================================
1      John      123     john@gmail.com

2      Joe      456       joe@gmail.com

3      .....

4   ....

6 ...

Well apparently id=5 is missing from our database.

1. What could be the reason for that? Bear in mind we are using @transactional services and dao.

2. Can spring data create new entries with these skipped ids? (configuration maybe?)

Answer 1

The id get 'reserved' when you issue an insert statement, but the insertion is performed until the transaction is commited; if an exception is thrown before that, the transaction gets rolled back, the insertion is not performed, so the reserved id is not used. All of this is assuming you're using auto-increments.

There might be other reasons, but this one in particular has happened to me a on ocassion, although is not really an issue to me.

Edit: this has happened to me on Postgres but I think it also happened on mysql; it's been a while since I last used it.

Also, you can check your JPA provider's the documentation on @GeneratedValues