MySQL Read Committed Snapshot alters the transaction behaviors running under Read committed isolation level.

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let us look at how to set up and manage Read Committed transaction levels.

Read Committed Transaction  isolation level

The MySQL Read Committed snapshot Isolation Transaction Level is the most effective tool to prevent dirty reads to keep the transaction clean from corrupted or uncommitted data. Non-repeatable and Phantom reads are more likely to show up in the transaction running on a particular level.

How to setup and enable read committed snapshot on SQL

Users can set up the Read committed Snapshot on SQL manually from the properties on the database.Under the options find the Read Committed Snapshot On. The default set option is Off.

As an alternative method User can type in ALTER DATABASE SET READ_COMMITTED_SNAPSHOT ON for activating the transaction level. Here the is the name of the database, for example, ‘ ALTER DATABASE MyDatabase SET READ_COMMITTED_SNAPSHOT ON.

The user can choose the databases to enable the transaction level. The default database names include BPMDB for the Process server database, PDWDB for the Performance Data Warehouse database, and CMNDB for the common databases.

Read committed isolation level: Read committed Snapshot is off

This Isolation level prohibits statements from reading modified data that are not yet committed by the other transactions. Read Committed Snapshot is by default Off. Shared Lock ensures the enforcement of the default isolation level.

The Shared lock prevents transactions from modifying the rows read by the current transaction. The Locking mechanism eliminates dirty reading to a huge extent and reduces concurrency for the transaction to complete its process.

Read committed isolation level: Read committed Snapshot is on

A Snapshot of the Data which is transitionally consistent is generated for each statement avoiding the necessity to place locks.  In this case, the  SQL server engine uses row versioning. Let’s go through a model situation for understanding the practical application. Begin by creating the environment as shonw below.

USE master
GO
CREATE DATABASE TestDB
GO
USE TestDB
GO
CREATE TABLE TestTable
(
ID INT,
Val CHAR(1)
)
INSERT INTO TestTable(ID, Val)
VALUES (1,'A'),(2,'B'),(3, 'C')

Read committed Snapshot is off

We have the TestDB database and the testable. The isolation level is the default- read committed and theread_committed_ snapshot is Off. Open up two query windows in the SQL Management studio.On the First window type in the following code.

--1.1
USE TestDB
GO
BEGIN TRANSACTION
UPDATE TestTable
SET val='X'
WHERE val= 'A'
WAITFOR DELAY '00:00:07
COMMIT

Type in the following code on the second window

--1.2
USE TestDB
GO
SELECT*FROM TestTable

After Executing the first query, execute the second query immediately. The results will open up as given below. Fro the first row.

--1.1
USE TestDB
GO
BEGIN TRANSACTION
UPDATE TestTable
SET val='X'
WHERE val= 'A'
WAITFOR DELAY '00:00:07
COMMIT
(1 row affected)

And, for the second row.

--1.2
USE TestDB
GO
SELECT*FROM TestTable
ID Val
1 1 X
2 2 B
3 3 C

The second query holds back till the first transaction is completed to return the modified results.

Read committed Snapshot is on

In the previous example, we have turned off the read_committed_snapshot. For this stage, we have to turn it on. Type in the statement given below to initiate the activation.

ALTER DATABASE TestDB SET READ_COMMITTED_SNAPSHOT ON

Next, as in the previous example open up two query windows. On the first window copy or type in the code given below.

--2.1
USE TestDB
GO
BEGIN TRANSACTION
UPDATE TestTable
SET Val='A'
WHERE Val='X'
WAITFOR DELAY '00:00:07'
COMMIT

And Paste the following on the second window

–2.2
USE TestDB
GO
SELECT * FROM TestTable

Execute the first and the second query in that order. In this case, the result of the second query won’t hold back for the first to complete and will immediately return the result. For the first row:

2.1
USE TestDB
GO
BEGIN TRANSACTION
UPDATE TestTable
SET Val='A'
WHERE Val='X'
WAITFOR DELAY '00:00:07'
COMMIT
(1 row affected)

For the second row

–2.2
USE TestDB
GO
SELECT * FROM TestTable
ID Val
1 1 X
2 2 B
3 3 C

Here we can note that the data is shown before the first query made its changes because the snapshot took the action and the locks didn’t prevent the second statement from reading its data.

Shared locks when Snapshot is on

We can use the Shared locks even if the READ COMMITTED SNAPSHOT is ON by using the READCOMMITTEDLOCK table hint instead of row visioning. Type in or copy the code given below on the first window:

--2.3
USE TestDB
GO
BEGIN TRANSACTION
UPDATE TestTable
SET Val='X'
WHERE Val='A'
WAITFOR DELAY '00:00:07'
COMMIT

And type in the second query as given below.

–2.4
USE TestDB
GO
SELECT * FROM TestTable WITH (READCOMMITTEDLOCK)

And the result will be as shown below. For the first row:

--2.3
USE TestDB
GO
BEGIN TRANSACTION
UPDATE TestTable
SET Val='X'
WHERE Val='A'
WAITFOR DELAY '00:00:07'
COMMIT
(1 row affected)

The results for the second row will be as shown below.

–2.4
USE TestDB
GO
SELECT * FROM TestTable WITH (READCOMMITTEDLOCK)
ID Val
1 1 X
2 2 B
3 3 C

In this case, the second query will wait for the first one to complete and return the result. Note that the Read committed snapshot only concentrates on changing the behavior of the transactions under the READ COMMITTED isolation level. The Second statement will wait for the first query to complete if we use it under the SERIALIZABLE or REPEATABLE READ isolation level.

Type in the following statement in the first query.

--3.1
USE TestDB
GO
BEGIN TRANSACTION
UPDATE TestTable
SET Val='A'
WHERE Val='X'
WAITFOR DELAY '00:00:07'
COMMIT

The SERIALIZABLE isolation will be used under the second query as shown below.

--3.2
USE TestDB
GO
SET TRANSACTION ISOLATION LEVEL SERIALIZABLE
SELECT * FROM TestTable

So in this case the second query will wait for the first to complete the process. The result of the first row is shown below.

--3.1
USE TestDB
GO
BEGIN TRANSACTION
UPDATE TestTable
SET Val='A'
WHERE Val='X'
WAITFOR DELAY '00:00:07'
COMMIT
(1 row affected)

The result of the second row is shown below.

--3.2
USE TestDB
GO
SET TRANSACTION ISOLATION LEVEL SERIALIZABLE
SELECT * FROM TestTable
ID Val
1 2 A
2 2 B
3 3 C

Read committed snapshot and behavioral pattern

The READ COMMITTED SNAPSHOT option is not an isolation level. It changes the behavior of the READ COMMITTED transaction isolation levels. We can increase the concurrency by enabling the READ COMMITTED SNAPSHOT = ON option, which is one of the major advantages of the option.

Note that the Snapshots will eat up a fair share of space as they are stored in the tempd database. We can check whether the Snapshot is active or not by typing in the following code.

SELECT name AS DatabaseName,is_read_committed_snapshot_on
FROM sys.databases
WHERE name = ‘TestDB'

Note that the, activation of the snapshot is depicted with ‘1’. We can change the stage to default or OFF by typing in the following.
SELECT name AS DatabaseName,is_read_committed_snapshot_on
FROM sys.databases
WHERE name = 'TestDB'

Here the value will be ‘0’ denoting that the Snapshot is Off or is in its default state.

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conclusion

To conclude, MySQL read committed snapshot is an effective tool for dirty read elimination. The MySQL read committed snapshot helps to keep the transaction from corrupted data.