QDatabase::transaction()

[sunil.thaha]

How do we know that the trasaction function is called ?
And is there any side effects if It's called more than once as shown below

QSqlDatabase* db = QSqlDatabase::database();
    db->transaction();
    // ......
    // ......
    db->transaction();
    // ......
    // ......
    db->rollback();

QSqlDatabase* db = QSqlDatabase::database();
    db->transaction();
    // ......
    // ......
    db->transaction();
    // ......
    // ......
    db->rollback();

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