What is Page Split?

Whenever you update existing rows with the data that is bigger in size to save on their data page then Page Split operation occur to make space for this new update. Data from existing data page first move to new page that is added during Page Split operation and make room to accommodate new records. That is why this operation is resource intensive and create fragmentation. Page splits only occur when data changes in the database. If you are adding all your data at the end of the table then page split will not occur.

If your fill factor is 100 or 0, it will work in a same way. That means there is no room in leaf level page to accommodate future updates. Setting fill factor value to 0 or 100 will increase page split occurrences.

How to Check Page Split in SQL Server?

Here, i will explain how to check page split in SQL Server. Whenever Page split occurs, it will be logged in transaction log file as LOP_DELETE_SPLIT operation. We use fn_dblog function to read SQL Server transaction log file. Run below T-SQL code to see all page splits occurrence using transaction log file.

SELECT *
FROM
fn_dblog (NULL, NULL)
WHERE
[Operation] = N'LOP_DELETE_SPLIT'

You can see LOP_DELETE_SPLIT is logged under operations column. That means it’s a page split operation.

Paul Randel has given very good sql code to check total number of page splits occurrence. Below is the code you need to run to get this details.

SELECT
[AllocUnitName] AS N'Index',
(CASE [Context]
WHEN N'LCX_INDEX_LEAF' THEN N'Nonclustered'
WHEN N'LCX_CLUSTERED' THEN N'Clustered'
ELSE N'Non-Leaf'
END) AS [SplitType],
COUNT (1) AS [SplitCount]
FROM
fn_dblog (NULL, NULL)
WHERE
[Operation] = N'LOP_DELETE_SPLIT'
GROUP BY [AllocUnitName], [Context];
GO

You can check total number of page splits are shown for each indexes in below screenshot. Now we will discuss how to avoid page split in SQL Server for such indexes.

How to Reduce Page Split Occurrences?

An index that has many random inserts and has very full pages will have an increased number of page splits. This causes more fragmentation and that is very bad for performance. We can reduce page splits by keeping some room in the data pages by setting fill factor value less than 100. The extra bytes on each page will help to minimize page splits caused by extra length in the rows. Read attached article to understand the best value for fill factor to avoid such page-split operations.

We can also use index rebuild operation with the FILL FACTOR option that will allow the page fullness to be changed to fit the query pattern on the index. This will also helpful in reducing total page-split counts.
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