The principle of starting and shutting down oracle-transaction commit or rollback
4. Transaction
4.1 Concepts of transactions
Starting with the first DML statement and ending with rollback or commit, all previous DML operations (insert,update,delete) fall within the scope of this transaction.
4.2 commit operation of transaction
(1) first unlock a user and change the password
SQL > alter user scott account unlock
User altered.
SQL > alter user scott identified by scott
User altered.
(2) Connect to scott users
SQL > conn scott/scott
Connected.
(3) create a tt table
SQL > create table tt (
2 id number
3)
SQL > insert into tt values (1)
1 row created.
SQL > select * from tt
ID
-
one
SQL > insert into tt values (2)
1 row created.
SQL > select * from tt
ID
-
one
two
SQL > commit;-- this is the end of a transaction
Commit complete.
This is another business.
SQL > update tt set id=3 where id=2
1 row updated.
SQL > select * from tt
ID
-
one
three
SQL > rollback;-- this transaction ends here
Rollback complete.
SQL > select * from tt
ID
-
one
two
Two questions:
(1)。 Will the committed transaction be written to the disk?
Not necessarily? Because the read and write of the system is asynchronous, the submitted data may still be in the dirty block list until there are enough dirty blocks.
The DBWn process will write a batch of dirty block information to disk.
(2)。 Is it true that transactions that are not committed will not be written to disk
Not necessarily? Because the space of the dirty block linked list is limited, if a large number of changes are made to the data at once, a large number of dirty blocks will exceed the dirty blocks.
The length of the linked list. At this point, the DBWn process will write the dirty block information to disk. If the user submits it, the data will already be written to disk.
If the user rolls back, there is no problem. Oracle will use the undo tablespace to roll back the broken old image to send the previous letter
The information is restored to disk.
