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Wikipedia defines following terms:

  • With a lock-based concurrency control DBMS implementation, serializability isolation level requires read and write locks (acquired on selected data) to be released at the end of the transaction. Also range-locks must be acquired when a SELECT query uses a ranged WHERE clause, especially to avoid the phantom reads phenomenon.
  • In repeatable reads isolation level, a lock-based concurrency control DBMS implementation keeps read and write locks (acquired on selected data) until the end of the transaction. However, range-locks are not managed, so phantom reads can occur.

As long as I interpret it correctly, both serializability and repeatable read isolation levels corresponds to rigorous two phase locking (in which both shared and exclusive mode locks are held till transaction commits/aborts), in which transactions are serializable by their commit order, according to the book by Korth et al..

However, book by Korth et al also says following repeatable read isolation level:

  • Repeatable read allows only committed data to be read and further requires that, between two reads of a data item by a transaction, no other transaction is allowed to update it.
  • However, the transaction may not be serializable with respect to other transactions. For instance, when it is searching for data satisfying some conditions, a transaction may find some of the data inserted by a committed transaction, but may not find other data inserted by the same transaction.

My doubt is, if rigorous 2PL schedules are serializable by commit order of its transaction, then why book by Korth et al says repeatable read isolation level may not ensure serializability?

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  • $\begingroup$ As what you've quoted states, REPEATABLE READ does not take range locks. You can't take a row lock on a row that isn't there, which is what the case is in a phantom read scenario. $\endgroup$ – Derek Elkins Mar 13 at 22:13

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