We only had two days so time
was short. I got an email reminder today. So this is the first blog that
will answer some questions that I didn’t get to cover in depth in class, and I
thought it would be an Excellent Question of the Day!
The Question that I got was “What is the
T-SQL SAVE TRANSACTION statement used for?”
The short answer is it is a way of setting a save point in
multiple Nested Explicit Transactions.
For the Long answer we’ll look a little deeper.
REFRESHER
Back in a little over two years ago I wrote a blog, We
Should Have A Talk: Your Database is Using ACID. If you are unfamiliar with ACID go read up on
it, because it is fundamental to what we are doing here.
In that same blog I talk about Explicit, Implicit, and
Nested Transactions. In case you miss
the ACID reading let’s cover those.
Every Transaction in SQL Server is an Implicit Transaction, unless otherwise specified. You can turn off Implicit Transactions but
that’s a talk for another time. Implicit
means implied, rather than expressly stated.
As with the picture we don’t have to say it. Hopefully things end a little better for us.
SQL Server works the same way. No, not like a cat driving a car. Transactions are just as Implicit.
Every single ALTER TABLE, CREATE, DELETE,
DENY, DROP, FETCH, GRANT, INSERT, OPEN, REVOKE, SELECT, TRUNCATE TABLE, or
UPDATE is a going to be wrapped in a BEGIN TRANSACTION and end with either a
COMMIT or a ROLLBACK.
Explicit means fully and clearly expressed or demonstrated;
leaving nothing merely implied.
There is no mistake about it. The following SQL Statement is an Explicit Transaction.
BEGIN TRANSACTION
INSERT INTO dbo.myTable1 DEFAULT VALUES
COMMIT TRANSACTION
So we’ve covered Implicit
and Explicit, now we get to Nested.
Nested Transactions are
multiple levels of Explicit
Transactions.
For Example:
Here we have 3 levels.
However, if any one of them fail or ROLLBACK
then they all Rollback.
Some
companies use Explicit Transactions as part of their error handling. Nothing wrong with that, but wouldn’t it be
nice if you could commit part of the statement, even if a later part fails?
SAVED!
Saved Transactions allow us to save our success as we go in
Nested Transaction, so that a Rollback doesn’t wipe out all of our
progress.
Depending how your code works,
you MIGHT be able to use this. Test the logic very soundly.
So let’s look at how this works. First we’ll create a Table to test out our
transaction.
if exists(select name from sys.tables where name='myTable1')
begin
drop table dbo.myTable1
end
go
create table dbo.myTable1(
myid int identity(1,1)
,mychar char(50)
,mychar2 char(50)
,constraint pk_myid_mytable1 primary
key clustered(myid)
)
Now let’s create a transaction to insert some data and roll
it all back.
BEGIN TRANSACTION
INSERT INTO dbo.myTable1(mychar, mychar2)
VALUES('some data 1', 'some more data 1')
BEGIN TRANSACTION
INSERT INTO dbo.myTable1(mychar, mychar2)
VALUES('some data 2', 'some more data 2')
BEGIN TRANSACTION
UPDATE dbo.myTable1
set mychar='some new data 1'
WHERE myid=1
BEGIN TRANSACTION
SELECT * FROM dbo.myTable1
COMMIT TRANSACTION
COMMIT TRANSACTION
COMMIT TRANSACTION
ROLLBACK TRANSACTION
Here’s the Output from our SSMS Windows.
/*
OOPS did I say ROLLBACK!?
*/
SELECT * FROM dbo.myTable1
We had a ROLLBACK at the end. So the work isn’t going to stick. Sure enough another select from the table and
everything is gone.
Now let’s try the
same code again, but now we’ll create a SAVE TRANSACTION and name is
testSavePoint. Note the syntax change. *We
need to call this in the ROLLBACK otherwise the whole thing will ROLLBACK
again.* ACID properties are still in play, so we need a COMMIT AFTER the ROLLBACK
/*
Let's Try this again
*/
BEGIN TRANSACTION
INSERT INTO dbo.myTable1(mychar, mychar2)
VALUES('some data 1', 'some more data 1')
SAVE TRANSACTION testSavePoint
BEGIN TRANSACTION
INSERT INTO dbo.myTable1(mychar, mychar2)
VALUES('some data 2', 'some more data 2')
save transaction testSavePoint
BEGIN TRANSACTION
UPDATE dbo.myTable1
set mychar='some new data 1'
WHERE myid=3
BEGIN TRANSACTION
SELECT * FROM dbo.myTable1
COMMIT TRANSACTION
COMMIT TRANSACTION
COMMIT TRANSACTION
ROLLBACK TRANSACTION testSavePoint
COMMIT TRANSACTION
Here’s our output from SSMS again. This looks very similar to last time with
only changes in the identity key.
/*
OOPS did I say ROLLBACK AGAIN!?
But did we save anything
*/
SELECT * FROM dbo.myTable1
This time however, only our Update Statement was rolled
back.
“So Balls,” you
say, “That’s cool, but will we actually
use this? If we are using Nested
Transactions would we only want to save ½ of it?”
Excellent Question Dear Reader! Let me answer that with a real world
scenario.
I was working with some friends earlier this year. We changed an update job that ran for hours
from row-by-row logic to set based logic.
The result was minutes instead of hours.
There were two parts of this job that we worked on.
It was a four step process.
First a range of data was selected from one table. Second we looked for missing data and
inserted anything that was missing based on matching natural keys. Third we then looked at everything that
matched and updated any rows that needed the information. Fourth we updated the range selected in another
table. We needed all four steps to
run. A Saved Transaction wouldn’t work
for us using their current logic.
However the job failed when we first ran it, because there
was an error on saving the range table.
Insert and Updates worked just fine.
The job would look at the range table and try the same range. Even though it was failing the job was
reporting success because the error handling did not catch the rollback. It was explicitly called, but executed on a
GO TO on error statement.
While we would have still had to fix the error on the range
table, we could have used SAVE TRANSACTION to save the insert and update. Then moved the range table update to its own
Transaction that would have better error handling. It also would save retry operations on the
job by not processing expensive Updates, Inserts, and Rollbacks.
Alright Dear Reader, there it is. SAVE TRANSACTION and how it works with Explicit
Transactions. Thanks for the questions,
keep them coming!
As always Thanks for stopping by.
Thanks,
Brad
