SQL Fool Adventures in SQL Tuning – a blog for the rest of us

Page Splitting & Rollbacks

So while I was at the grocery store last night, my mind wandered to SQL Server. This time, I was pondering what happens to a page split if the transaction is rolled back. I didn't know the answer, but my guess was that the page split remained, since it would be less expensive for SQL Server to leave the data where it was sitting. Also, in theory, if the page split occurred once, it could need to occur again, so why undergo that same expense twice?

I decided to run a simple test to see what would happen. First, I created a really wide table and tossed 4 rows into it:

Create Table myTable
(
      id            int Primary Key
    , wideColumn    char(2000) 
);
 
Insert Into myTable
Select 1, Replicate('A', 2000) Union All
Select 2, Replicate('B', 2000) Union All
Select 4, Replicate('C', 2000) Union All
Select 5, Replicate('D', 2000);

I left an open spot for id=3, so I can force a page split. Next, I looked at the page data using the script I posted in February.

Here's what I saw:

Before

Pay attention to the 2 items boxed in red. m_slotCnt tells us how many records are on the page, and m_nextPage tells us the address of the next page. As you may have guessed, a m_nextPage value of 0:0 means you're looking at the last page allocated to the object.

Now let's insert a record, roll it back, and see what happens:

Begin Transaction;
 
Insert Into myTable
Select 3, Replicate('E', 2000);
 
Rollback Transaction;

I ran my DBCC Page command again and here's what I saw:

After

As you can see, m_slotCnt is now 2, and m_nextPage is no longer 0:0 (although your actual page number will probably be different than mine). If I pull up the new page, I find my 2 relocated records, id's 4 and 5.

So what's this all mean? In short, page splits are NOT reversed when a transaction is rolled back. Why should you care? Well, you probably wouldn't care much, unless you roll back a lot of transactions. But this is also a good thing to keep in mind if you have to abort a really large insert or update; if you don't plan to re-execute the aborted script, you may want to defrag your indexes to fix the splits.

Source: http://sqlfool.com/2009/04/page-splitting-rollbacks/