READ_COMMITTED_SNAPSHOTdatabase options enable/disable the use of row versioning. These two options are often discussed independently and collectively due to their behaviors, and may have resulted some confusion on how these two database options correlate and affect each other on transaction isolation level.
I have came across multiple forum and blog online that suggest having both
READ_COMMITTED_SNAPSHOTdatabase options set to ON whenever someone asks for assistance and recommendation regarding snapshot, row versioning or similar question.
Perhaps the word SNAPSHOT in both setting causes some confusion.
Although these two isolation level utilize row versioning, they should be independent from each other setting. Let's do some test to evaluate how these two database options affect the transaction isolation level,
CREATE DATABASE TEST; GO CREATE TABLE TABLE1 ( ID tinyint, Details varchar(10) ); GO INSERT INTO TABLE1 VALUES (1, 'Original'); GO SELECT name, snapshot_isolation_state_desc, is_read_committed_snapshot_on FROM sys.databases WHERE name = 'TEST'; GO
First test with both settings confirmed to be OFF.
USE TEST; BEGIN TRAN UPDATE TABLE1 SET Details = 'Update' WHERE ID = 1; --COMMIT; --ROLLBACK; GO
USE TEST; SELECT ID, Details FROM TABLE1 WHERE ID = 1; GO
In this test, query 2 is waiting for query 1 to commit, dm_tran_locks DMV shows that exclusive lock on TABLE1 incurred by query 1.
USE TEST; SELECT DB_NAME(tl.resource_database_id) AS DBName, resource_type, OBJECT_NAME(resource_associated_entity_id) AS tbl_name, request_mode, request_status, request_session_id FROM sys.dm_tran_locks tl WHERE resource_database_id = db_id('TEST') AND resource_type = 'OBJECT'
Second test, rollback previous transaction, set
READ_COMMITTED_SNAPSHOT ONbut leave
ALTER DATABASE TEST SET READ_COMMITTED_SNAPSHOT ON WITH ROLLBACK IMMEDIATE; GO
Run Query 1, and run query 2. DMV shows query 1 incur exclusive lock, but query 2 returns details with 'Original' without query 1 commit the transaction. It appears that
READ_COMMITTEDrow versioning is in place.
SET TRANSACTION ISOLATION LEVEL SNAPSHOT;on query 1 and query 2, and run query 1 or query 2 returns error - Snapshot isolation transaction failed accessing database 'TEST' because snapshot isolation is not allowed in this database. Use ALTER DATABASE to allow snapshot isolation.
Third test, rollback previous transaction. Set
ALTER DATABASE TEST SET READ_COMMITTED_SNAPSHOT OFF WITH ROLLBACK IMMEDIATE; GO ALTER DATABASE TEST SET ALLOW_SNAPSHOT_ISOLATION ON; GO
Run query 1, and then query 2. DMV shows exclusive lock incurred by query 1. Query 2 appears to be waiting for query 1 to complete. Turning
ALLOW_SNAPSHOT_ISOLATION ONdoesn't appear to enable
READ COMMITTEDrow versioning.
SET TRANSACTION ISOLATION LEVEL SNAPSHOT;to both query 1 and query 2. Run query 1 and then query 2. While DMV shows query 1 incur exclusive lock, query 2 return details with 'Original'. Snapshot isolation appears to be in place.
The test above shows that in order for database engine to use row versioning instead of locks for
READ_COMMITTEDdefault behavior, the database
READ_COMMITTED_SNAPSHOTis set to ON regardless of what
ALLOW_SNAPSHOT_ISOLATIONsetting is set to ON only to allow snapshot isolation when starting a transaction (e.g.
SET TRANSACTION ISOLATION LEVEL SNAPSHOT) regardless of
The only reason to have these two options set to ON is when the intention is to have both
READ COMMITTEDrow versioning AND snapshot isolation level.