In SQL Server, the placement order of a table’s records in memory pages is determined by the clustered index. When new records are inserted, they are added to the memory page in the order they belong to. However, there are cases where page splits occur, which can impact performance.

A page split happens when records from one memory page are moved to another page during changes to the table. This occurs when there is not enough room in the current memory page to accommodate a new record. As a result, some records need to be shifted around, and the page split occurs when a record is moved to a different page.

Page splits are considered bad for performance, and there are techniques to reduce or eliminate the risk of page splits. One way to minimize page splits is to carefully choose the clustered index key. It’s important to select a key that has a low likelihood of causing frequent page splits.

When a primary key is created in SQL Server, it also creates an index, which is by default clustered. The data in the table is physically ordered by the primary key. This means that when records are inserted, they are inserted in the order of the primary key value.

Let’s consider an example. We have a table called HumanResources.Contractor with a clustered index on the SSN (Social Security Number) field. When the first three records are inserted into the table, they all occupy the same memory page. However, when a fourth record is inserted, a page split occurs because there is not enough room in the current page. The records are shifted to make room for the new record to be inserted in the proper sequence according to the clustered field value.

It’s important to note that as more records are inserted, the likelihood of page splits increases. This can impact performance, especially if page splits occur frequently. Therefore, it’s crucial to monitor and optimize the clustered index key to minimize page splits.

It’s worth mentioning that this article provides a basic explanation of page splits and does not delve into the details of how rows are ordered and B-trees are organized. For a more in-depth understanding, you can refer to the MSDN page on this topic.

Remember, page splits can have a significant impact on SQL Server performance, so it’s essential to be aware of their occurrence and take steps to minimize them.

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