In this post, I am sharing a demonstration in which you can get to know that committed inner transactions are not releasing any log disk space in SQL Server.

One of our developers was telling me like when we commit the transaction it will release the occupied log space, but it is not entirely correct.
If you are dealing with nested transactions, it is true only for outer committed transactions.

Please check the below demonstration of this.

Create a sample database:

CREATE DATABASE Test_Tran;
GO
USE Test_Tran
GO

Check the log size:

CHECKPOINT
GO
DBCC SQLPERF ('LOGSPACE')
GO
-- the result
-- 4.557673

Create a sample table:

CREATE TABLE tbl_TestTrans(ID INT)
GO

Open outer and inner transaction and insert few sample records:

BEGIN TRANSACTION OuterTrans
GO
	BEGIN TRANSACTION InnerTrans
	GO
		INSERT INTO tbl_TestTrans VALUES(88)
		GO 1000
	COMMIT TRANSACTION InnerTrans
	GO

Check the log size:
The inner transaction is committed, but log size is increased, and it didn’t release the logs.

CHECKPOINT;
GO
DBCC SQLPERF ('LOGSPACE');
GO

-- the result:
-- 8.767107

Now, commit outer transaction:

COMMIT TRANSACTION OuterTrans
GO

Check the log size:
Once you commit your outer transaction, it releases the log size.

CHECKPOINT;
GO
DBCC SQLPERF ('LOGSPACE');
GO

-- the result:
-- 5.08

I hope you know that we can create nested transactions in SQL Server. If you are giving a SQL Server developer interview, an interviewer may ask a question like If Outer transaction ROLLBACK, what happens to an Inner transaction in SQL Server.

The interviewer can also write code in paper and can ask a developer to solve it.

You can find the answer in below demonstration so test it yourself.

Create a sample table:

CREATE TABLE tbl_TestTrans(ID INT)
GO

Open outer and inner transaction and insert few sample records:

BEGIN TRANSACTION OuterTrans
GO
	BEGIN TRANSACTION InnerTrans
	GO
		INSERT INTO tbl_TestTrans VALUES(88)
		GO 1000
	COMMIT TRANSACTION InnerTrans
	GO

Check the count of your table:

SELECT COUNT(1) FROM tbl_TestTrans
GO
--The result is:
-- 1000

Now, Rollback your Outer transactions:

ROLLBACK TRANSACTION OuterTrans
GO

Now, Check the count of your table:
Which is 0 now. Although you committed inner transaction for those 1000 records, outer transaction was not committed, so it rollbacked all transactions.

SELECT COUNT(1) FROM tbl_TestTrans
GO
--The result is:
-- 0