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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?

  • Because it allows phantom reads? Phantom reads mean that transaction T2 started after T1 but committed before T1, thus violating serializability. – mustaccio Mar 13 at 12:59
  • Definition of phantom reads from the Korth's book: "A transaction re-executes a query returning a set of rows that satisfy a search condition and finds that the set of rows satisfying the condition has changed as a result of another recently committed transaction". So because Repeatable read does not have range locks, phantom reads might occur. But I am unable to see how phantom reads can affect serializability. – anir Mar 13 at 13:38
  • [...continued from last comment] Is it like if the count of selected rows change and if the transaction uses this count to update database, then transactions wont be serializable since concurrent and any serial execution will have different effect on the database? – anir Mar 13 at 13:39

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