Below we focus on the ITL transaction slot. Each data block of Oracle has one or more transaction slots, and each concurrent access transaction to the data block occupies a transaction slot.
Each thing will ITL thing slot consists of slot number, XID, Uba, Flag, Lck, Scn/Fsc several parts.
Itl Xid Uba Flag Lck Scn/Fsc
0x01 0x0006.020.00000271 0x00800205.0257.13 C--- 0 scn 0x0000.001732c4
0x02 0x0008.006.00000279 0x00800351.0278.15 ---- 1 fsc 0x0000.00000000
ITL (interested transaction list) slot number.
XID (transaction id) Transaction ID, there is a record in the rollback segment transaction table corresponding to this transaction. Xid composition: Undo Segment Number +Transaction Table Slot Number+ Wrap .
UBA (undo block address) rollback segment address, the rollback segment address corresponding to the transaction. Uba composition: rollback block address (undo file number and data block number) + rollback sequence number + rollback record number .
SQL> select xidusn,xidslot,xidsqn,ubafil,ubablk,ubasqn,ubarec from v$transaction; XIDUSN XIDSLOT XIDSQN UBAFIL UBABLK UBASQN UBAREC ---------- ---------- ---------- ---------- ---------- ---------- ---------- 8 6 633 2 849 632 21
Flag: Transaction flag bit. This flag bit records the operation status of the transaction. The meanings of each flag are:
C = transaction has been committed and locks cleaned out -- things have been committed and locks have been cleared
B = this undo record contains the undo for this ITL entry
U = transaction committed (maybe long ago); SCN is an upper bound -- the transaction has been committed, but the lock has not been cleared
T = transaction was still active at block cleanout SCN -- the transaction was still active at block cleanout SCN It is active . If there is a committed transaction on the block, the block will be cleared during clean count, but the transactions in this block will not be cleared.
Lck: Indicates the number of rows affected by this transaction. We see that the transaction slot Lck of No. 01 is 0, because the flag of the transaction in the transaction slot is C, which proves that the transaction
has been submitted, the lock has been cleared, and the transaction slot can be reused. The thing slot Lck of No. 02 is 1, because I made an update to the first line and did not submit, and the Flag is ---- indicating that the thing is active.
Scn/Fsc: Commit SCN or Fast Commit Fsc SCN.
The row-level lock in each record corresponds to the Itl entry lb, which corresponds to the sequence number in the Itl list, that is, the lock generated by that transaction on that record.
Detailed reference: http://czmmiao.iteye.com/blog/1495332