What is Hibernate's flush behaviour and how does it affect SQL execution?

Flushing synchronises the in-memory persistence context with the database by sending pending SQL. Flush does NOT commit the transaction.

// AUTO flush mode (default):
Product p = new Product("Laptop", new BigDecimal("999"));
session.persist(p);  // INSERT scheduled, NOT sent yet

// Query triggers flush in AUTO mode:
Long count = session.createQuery("SELECT COUNT(p) FROM Product p", Long.class)
    .getSingleResult();  // INSERT flushed first so count is accurate

// Explicit flush:
session.flush();  // sends SQL but does NOT commit

// Check for pending changes:
session.isDirty();  // true if unflushed changes exist

// SQL ordering at flush: INSERT -> UPDATE -> DELETE
// Prevents FK constraint violations

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