The XACT_ABORT option: What it is and what it is used for, all explained in a simple way!

Hi Guys,
Welcome back!

After a dip in the maze of the logic of the SQL Server optimizer, today we talk about the command XACT_ABORT.
We will see how important this command is when we deal with transactions.

Do not worry, I will try to explain in the simplest way possible!

XACT_ABORT

The XACT_ABORT is an option which can have only two possible values: ON and OFF.
 

You can simply set it's value with the T-SQL command:

       
SET XACT_ABORT ON
OR 
       
SET XACT_ABORT OFF


If XACT_ABORT is set to ON then if a T-SQL command throws an error then the transaction will be rollbacked.

Suppose of having a trigger triggered by an update on the table A
If an istruction inside the trigger fail then also the update on the table will be rollbacked.

Yes, this is the concept.

Let's have this example with XACT_ABORT set to ON

Suppose we have a TABLE_A table with the field CAMPO1 non nullable with a 0 value.

       
SET XACT_ABORT ON
 
BEGIN TRAN
UPDATE TABLE_A SET CAMPO1 = 1 WHERE ID = 1  ($)
COMMIT

TRIGGER ON UPDATE ON THE TABLE_A
BEGIN
   UPDATE TABLE_A SET CAMPO1 = NULL WHERE ID = 1  (*)
END

The update (*) fail because CAMPO1 does not accept the null.
The transation is rolled back and so the update ($) in canceled.

Q: What is the value of the column CAMPO1?
A: The value is 0 because the update ($) is never happened!

And if XACT_ABORT is set to OFF?

Well, it depends!

In some cases only the T-SQL statement that raised the error is rolled back but not the entire transaction.
The behavior depend from the severity of the error.

Let's try.. 

SET XACT_ABORT OFF
 
BEGIN TRAN
UPDATE TABLE_A SET CAMPO1 = 1 WHERE ID = 1  ($)
COMMIT

TRIGGER IN UPDATE ON THE TABLE_A
BEGIN
    UPDATE TABLE_A SET CAMPO1 = NULL WHERE ID = 1  (*)
END

As before the update (*) fail because CAMPO1 does not accept the null.

Only the T-SQL update is rolled back.
The update ($) in commited

What is the value of the column CAMPO1?
The value is 0 because the update ($) is never happened!

Q: What is the value of the column CAMPO1?
A: The value is 1 because the update ($) is committed


It's not really easy? Yes i suppose!

Just remeber that the THROW statement honors SET XACT_ABORT.
While then RAISERROR command does not.
So use THROW instead of RAISERROR.


That's all for today.
I wish you an amazing saturday.

See you soon,
Luca


Luca Biondi @ SQLServerPerformance blog 2020!











Previous post: Inside the SQL Server Query Optimizer - part 6 What is the memo?

Comments

Post a Comment

I Post più popolari

SQL Server, execution plan and the lazy spool (clearly explained)

Image
Hi Guys, Welcome back! In one of the last posts we talked about Table Spool . We had explained what Spool are for by telling the somewhat curious story of the halloween problem . Here we specifically talked about a type of spool called Eager Spool   You can look to the execution plan below where the eager Spool is used to avoid the halloween problem in classic query of the "salary increase".. i suggest Well, if the story of the eager spool intrigued you, today instead we will speak about another type of Spool, the Lazy Spool . The Lazy spool Let's take a small step back. So, what is a Spool operation?  As we have already understood a spool operation is simply a temporary storage of data . Data are read from a source table and stored inside a worktable in the Tempdb database . While an Eager Spool is used to prevent the Halloween problem, the Lazy Spool is instead used to store data that will later be needed again when running a query . So, when a Lazy spool is used?  ...
Read more

SQL Server, datetime vs. datetime2

Image
Hi guys, This time, in this post we will speak about datetime and datetime2 datatypes. We will talk about the pros and cons. We will talk about all the differences between the two types of data. Write me in the comments which type of data do you prefer between the two. Enjoy the reading!     Datetime Vs. Datetime2 First of all, it must be said that the datatype datetime is certainly the best known native format for storing a date in SQL Server. However, Microsoft has also been introducing a new type of data for many years now, also used to manage dates. It is in fact from SQL server 2008 that it is present in datetime2 data type. This "new" type of data was born primarily to have higher accuracy . Let's in fact talk about ... precision ! Datetime Certainly the datetime data type is not very precise and certainly there are many cases in which its precision is simply not sufficient. Let's see it with an example. Open the SQL Management studio (SSMS) and write the fo...
Read more

La clausola NOLOCK. Approfondiamo e facciamo chiarezza!

Image
Ehi ragazzi! La domanda è di quelle importanti! Siete pronti per scoprire tutto ma proprio tutto sulla clausola NOLOCK?   Se la riposta è si, allora seguitemi! Oggi ne spiegheremo il funzionamento in maniera approfondita. Faremo una introduzione al concetto di tipi di LOCK Ed infine vedremo come mai una SELECT ne può bloccare un'altra. Non vi nascondo che il motivo di questo articolo è fare chiarezza. Su questo argomento si sentono spesso affermazioni “curiose” ma tra le tante questa è la prima: “ uso la SELECT con l’opzione WITH(LOCK) così non blocco le altre tabelle ”.   Ma, è proprio vero quanto affermato? Beh oggi facciamo il punto!   Buona lettura!     La clausola NOLOCK Diciamo subito per iniziare che cos’è la NOLOCK. NOLOCK è il nome di una clausola che è possibile specificare all’interno di uno statement di tipo SELECT utilizzando la seguente sintassi: SELECT * FROM TABELLA WITH (NOLOCK) Si noti che non possiamo ...
Read more