persist() Doesn't Always Wait for flush()

"The INSERT runs on flush or commit" is the usual rule, and it's true for one of these two entities and false for the other — with no difference in the code that calls persist().

java

@Id @GeneratedValue(strategy = GenerationType.IDENTITY)
Long id;   // on IdentityThing

@Id @GeneratedValue(strategy = GenerationType.SEQUENCE)
Long id;   // on SequenceThing

text

IDENTITY: statements right after persist(), before flush: 1
IDENTITY: statements after flush(): 1

SEQUENCE: statements right after persist(), before flush: 1
SEQUENCE: statements after flush(): 2

run in a container, real Hibernate statistics — identical calling code, different generation strategy

With GenerationType.IDENTITY, the database itself assigns the id — an auto-increment column — and there is no way to know what it is without actually running the INSERT. So persist() runs it immediately; flush() finds nothing left to do. With GenerationType.SEQUENCE, Hibernate can fetch the next id from a separate sequence up front — that's the one statement before flush — and defer the row's actual INSERT the way the general rule describes.