User can modify different rows of the table at the same time but cannot modify the same row at the same time. Example: to avoid truncate and alter operation during table modification. Table Level Lock (TM) – When Row lock (TX) is obtained an additional Table lock is also obtained to prevent any DDL operations to occur while a DML is in progress. Row level Lock (TX) – This obtains a lock on the particular row being modified and any other transaction attempting to modify the same row gets blocked, till the one already owning it finishes. for update) oracle obtains 2 locks on the table. When a DML is executed (update/delete/insert,merge, and select …. When the change occurs, the row is locked until the session either commits the change or rolls the change back. Oracle blocking locks mainly happens when a session issues an insert, update or delete command that changes a row. This will block the second until the first one has done its work. Oracle Blocking Sessions occur when one sessions holds an exclusive lock on an object and doesn’t release it before another sessions wants to update the same data.
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