Transactions, locks, isolation levels

Question

I have a few questions regarding subject from the title. First of all, lets assume that we work with JDBC, and there we have 2 transactions T1 and T2. In T1 we execute select st

Answer 1

Your question is a good one. Understanding what kind of locks are acquired allows a deep understanding of DBMS's. In SQL Server, under all isolation levels (Read Uncommitted, Read Committed (default), Repeatable Reads, Serializable) Exclusive Locks are acquired for Write operations.

Exclusive locks are released when transaction ends, regardless of the isolation level.

The difference between the isolation levels refers to the way in which Shared (Read) Locks are acquired/released.

Under Read Uncommitted isolation level, no Shared locks are acquired. Under this isolation level the concurrency issue known as "Dirty Reads" can occur.

Under Read Committed isolation level, Shared Locks are acquired for the concerned records. The Shared Locks are released when the current instruction ends. This isolation level prevents "Dirty Reads" but, since the record can be updated by other concurrent transactions, "Non-Repeatable Reads" or "Phantom Reads" can occur.

Under Repeatable Reads isolation level, Shared Locks are acquired for the transaction duration. "Dirty Reads" and "Non-Repeatable Reads" are prevented but "Phantom Reads" can still occur.

Under Serializable isolation level, ranged Shared Locks are acquired for the transaction duration. None of the above mentioned concurrency issues occur but performance is drastically reduced and there is the risk of Deadlocks occurrence.