By J. Lewis
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Additional resources for Oracle Core - Essential Internals for DBAs, Devels.
As a general principle, any sort of counter that is stored on the database is unlikely to be reset when the instance restarts. Remember, every slot in every undo segment has its own wrap#, so it would be a lot of work at startup to reset them all. Start and End of Transaction When a session starts a transaction, it picks an undo segment, picks an entry from the transaction table, increments the wrap#, changes the state to “active” (value 10), and modifies a few other columns. 2) that will ultimately get into the redo log file; this declares to the world and writes into the database the fact that the session has an active transaction.
The examples of dumps that I’ve printed so far came from an instance where nothing else was running, so when I ran the following queries, I knew they would return just one row which would be for the transaction I had started: select xidusn, xidslot, xidsqn from v$transaction; XIDUSN XIDSLOT XIDSQN ---------- ---------- ---------9 2 8211 select trunc(id1/65536) usn, mod(id1,65536) slot, id2 wrap, lmode from V$lock where type = 'TX'; USN SLOT WRAP LMODE ---------- ---------- ---------- ---------9 2 8211 6 You’ll notice that the lock mode on this “transaction lock” is 6 (exclusive, or X, mode).
Note You might wonder why Oracle doesn’t do a complete cleanout on commit. The reason is probably speed. If you know which ITL entry you’ve been using (and that’s part of the information the session keeps in its list of modified blocks), you can calculate exactly where in the buffered block you have to apply your changes. Remember, ITL entries do not move; on the other hand, the location of a row in a block can change, so the only way to clear a row lock byte is by following pointers—which could take more time (especially in the bad old days when CPUs had very small caches).