You can check the status of the READ_COMMITTED_SNAPSHOT setting using the sys.databases view. Check the value of the is_read_committed_snapshot_on column. Already asked and answered. As for the duration, Books Online states that there can’t be any other connections to the database when this takes place, but it doesn’t require single-user mode. So you may be … Read more
I want to understand the lock each transaction isolation takes on the table For example, you have 3 concurrent processes A, B and C. A starts a transaction, writes data and commit/rollback (depending on results). B just executes a SELECT statement to read data. C reads and updates data. All these process work on the … Read more
SELECT is_read_committed_snapshot_on FROM sys.databases WHERE name=”YourDatabase” Return value: 1: READ_COMMITTED_SNAPSHOT option is ON. Read operations under the READ COMMITTED isolation level are based on snapshot scans and do not acquire locks. 0 (default): READ_COMMITTED_SNAPSHOT option is OFF. Read operations under the READ COMMITTED isolation level use Shared (S) locks.
Run this: SELECT CASE transaction_isolation_level WHEN 0 THEN ‘Unspecified’ WHEN 1 THEN ‘ReadUncommitted’ WHEN 2 THEN ‘ReadCommitted’ WHEN 3 THEN ‘Repeatable’ WHEN 4 THEN ‘Serializable’ WHEN 5 THEN ‘Snapshot’ END AS TRANSACTION_ISOLATION_LEVEL FROM sys.dm_exec_sessions where session_id = @@SPID learn.microsoft.com reference for the constant values.
From Wikipedia (which has great and detailed examples for this): A non-repeatable read occurs, when during the course of a transaction, a row is retrieved twice and the values within the row differ between reads. and A phantom read occurs when, in the course of a transaction, two identical queries are executed, and the collection … Read more
This isolation level allows dirty reads. One transaction may see uncommitted changes made by some other transaction. To maintain the highest level of isolation, a DBMS usually acquires locks on data, which may result in a loss of concurrency and a high locking overhead. This isolation level relaxes this property. You may want to check … Read more
Read committed is an isolation level that guarantees that any data read was committed at the moment is read. It simply restricts the reader from seeing any intermediate, uncommitted, ‘dirty’ read. It makes no promise whatsoever that if the transaction re-issues the read, will find the Same data, data is free to change after it … Read more