Let’s look at the following script:
CREATE TABLE LoadMeFast (Id INT NOT NULL, InsertDate DATETIME NOT NULL, ABunchOfLetters NVARCHAR(100) ) GO SET NOCOUNT ON GO DECLARE @i INT = 1 WHILE @i <= 100000 BEGIN INSERT LoadMeFast SELECT @i, GETDATE(), REPLICATE(N'ABCD', 25) SET @i+=1 END GO
The script runs for more than 20 seconds. What do you think is the problem that causes the script to run that long? Our instinct is to say the reason is we run the script on a row-by-row basis and not as a set-based solution. This is a part of the truth, but not all of it.
Now let’s look at the following script:
TRUNCATE TABLE LoadMeFast GO BEGIN TRAN DECLARE @i INT = 1 WHILE @i <= 100000 BEGIN INSERT LoadMeFast SELECT @i, GETDATE(), REPLICATE(N'ABCD', 25) SET @i+=1 END COMMIT
The only difference between the scripts is the fact that the second script is executed inside an explicit transaction. Now let’s see how the Log Flushes/Sec performance counter looks:
The Real Reason:
Each transaction in SQL Server has to be durable on disk before been considered as committed. In other words, each transaction has to be written to the Transaction Log on disk before the process can move on. When we issue an insert/update/delete statement without an explicit transaction, SQL Server opens an implicit transaction for it. This means that we have to go to disk for each and every Insert statement inside the first While loop. That’s why we see so many log flushes in Performance Monitor for the first run.
In the second script, we handle the transaction on our own. SQL Server goes to disk on commit or when one (or more) of his 60K log blocks fill up. This means that our work with the disk is much more efficient, and we can see that under the second run arrow in Performance Monitor. The #1 enemy of data loading is the Transaction Log. Remember it and act accordingly.
Taking it to the Field
Now that we know how it works, we can take this knowledge to the field. We need to remember that a set-based solution is almost always more efficient performance wise, but it’s not always the case, and that solution is not always easy to write.
Recently, while examining a process at a client site, I spotted a cursor that went over a 200 rows table. Each row contained a few update statements that were executed using Dynamic-SQL, so we got to a total of about 2000 update statements that needed to be executed. Instead of trying to reduce the number of updates, which would make each update much more complex and cause the optimizer choose a suboptimal plan and potentially cause bugs, we just wrapped the cursor with a BEGIN TRAN and COMMIT, and performance was improved by hundreds of percents.
What to Watch Out For
When using this technique in the right places, your total process execution time will decrease, but you will put more locks on your tables. You need to verify you don’t trigger Lock Escalation that will block the whole table for other processes and kill your concurrency. You can do that by disabling Lock Escalation for the table, or by issuing a COMMIT and opening a new transaction every, let’s say, 2000 statements (Lock Escalation is triggered after about 5000 locks are held on the table). You will also potentially block each row for a bigger amount of time, and you need to verify you don’t hurt other processes that potentially need to access the rows.
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