Wait Event – enq: TX – row lock contention | Oracle Database Internal Mechanism

This wait event can occur for several reasons.

If one user is wanting to update or delete a row or rows that another session is modifying. The session holding the lock will release it when it performs a COMMIT or ROLLBACK.
If a session is waiting due to potential duplicates in a UNIQUE index. If two sessions try to insert the same key value, the second session has to wait to see if an ORA-0001 should be raised or not. The session holding the lock will release it when it performs a COMMIT or ROLLBACK.
If a session is waiting due to a shared bitmap index fragment. Bitmap indexes index key values and a range of rowids. Each entry in a bitmap index can cover many rows in the actual table. If two sessions want to update rows covered by the same bitmap index fragment, then the second session waits for the first transaction to either COMMIT or ROLLBACK by waiting for the TX lock.