Clearing ITL Slots in Oracle

For the DML operation, transaction keeps entry in the blocks containing the target rows. These entries are maintained by ITL(Interested Transaction List). Now our interest is in the question when this ITL entries will be cleared.

To answer that question, we can consider this scenario: a transaction updates 10000 records, on 10000 different blocks. Naturally there will be 10000 ITL slots, one on each block, all pointing to the same transaction ID. The transaction commits; and the locks are released. Should Oracle revisit each block and remove the ITL entry corresponding to the transaction as a part of the commit operation?

If that were the processing logic, the commit would have taken a very long time. Acquiring the buffers of the 10000 blocks and updating the ITL entry will not be quick; it will take a very long time, prolonging the commit processing. Target of the Oracle design  that the commit processing is actually very quick, with a flush of the log buffer to redo logs and the writing of the commit marker in the redo stream. Even a checkpoint to the datafiles is not done as a part of commit processing – all the effort going towards making the process fast, very fast. Had Oracle added the logic of altering ITL slots, the commit processing would have been potentially long, very long. Therefore Oracle does not remove the ITL entries after that transaction ends (by committing, or rolling back); the slots are just left behind as artifacts.

So, when does the ITL entry gets cleared? When block’s buffer is written to the disk, the unneeded ITL entries are checked and cleared out.