Transaction isolation levels relation with locks on table

Question

I have read about 4 levels of isolation:

Isolation Level       Dirty Read    Nonrepeatable Read  Phantom Read  
READ UNCOMMITTED      Permitted       Permitt         


        

Answer 1

The locks are always taken at DB level:-

Oracle official Document:- To avoid conflicts during a transaction, a DBMS uses locks, mechanisms for blocking access by others to the data that is being accessed by the transaction. (Note that in auto-commit mode, where each statement is a transaction, locks are held for only one statement.) After a lock is set, it remains in force until the transaction is committed or rolled back. For example, a DBMS could lock a row of a table until updates to it have been committed. The effect of this lock would be to prevent a user from getting a dirty read, that is, reading a value before it is made permanent. (Accessing an updated value that has not been committed is considered a dirty read because it is possible for that value to be rolled back to its previous value. If you read a value that is later rolled back, you will have read an invalid value.)

How locks are set is determined by what is called a transaction isolation level, which can range from not supporting transactions at all to supporting transactions that enforce very strict access rules.

One example of a transaction isolation level is TRANSACTION_READ_COMMITTED, which will not allow a value to be accessed until after it has been committed. In other words, if the transaction isolation level is set to TRANSACTION_READ_COMMITTED, the DBMS does not allow dirty reads to occur. The interface Connection includes five values that represent the transaction isolation levels you can use in JDBC.