Beginning Oracle Database 11g Administration From Novice to Professional

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CHAPTER 11 FIXING PROBLEMS
ORA-00060: Deadlock Detected
Once again, we see only a minimal explanation in Oracle Database 11g Error Messages:
This error is frequently encountered but is one of the least understood Oracle errors.
A deadlock is a situation in which two transactions interfere with each other; each is waiting
for a lock on a resource locked by the other. The correct thing for the application to do
when this error is received is to issue the command and retry the transaction.
Create two tables, and , related to each other by a referential constraint
(foreign key), and populate them with data by using the following commands:
Suppose that two users try to delete the data from the table, but each deletes
records in a different order. At step 3 in the following example, user A will be blocked
because user B has already locked the requested record. User B will be blocked in turn at
step 4, and a deadlock results. To resolve the deadlock, Oracle picks either user A or user
B as a victim, and any effects of the last command issued by the victim are rolled back.
To allow the other user to proceed, the victim must then issue the command to
release any other locks it still holds—it can then retry its transaction: