Oracle - How does Oracle manage transaction specific DML statements

Question

Imagine I have this simple table:

Table Name: Table1
Columns:    Col1 NUMBER (Primary Key)
            Col2 NUMBER

If I insert a record into Ta

Answer 1

Oracle creates an index to enforce the primary key constraint (a unique index by default). When Session A inserts the first row, the index structure is updated but the change is not committed. When Session B tries to insert the second row, the index maintenance operation notes that there is already a pending entry in the index with that particular key. Session B cannot acquire the latch that protects the shared index structure so it will block until Session A's transaction completes. At that point, Session B will either be able to acquire the latch and make its own modification to the index (because A rolled back) or it will note that the other entry has been committed and will throw a unique constraint violation (because A committed).