Beginning Microsoft SQL Server 2008 ... - S3 Tech Training

Page Splits — A First Look
All of this works quite nicely on the read side of the equation — it’s the insert that gets a little tricky.
Recall that the B in B-Tree stands for balanced. You may also recall that I mentioned that a B-Tree is balanced
because about half the data is on either side every time you run into a branch in the tree. B-Trees
are sometimes referred to as self-balancing because the way new data is added to the tree generally prevents
them from becoming lopsided.
When data is added to the tree, a node will eventually become full and will need to split. Because in SQL
Server, a node equates to a page, this is called a page split, illustrated in Figure 9-3.
When a page split occurs, data is automatically moved around to keep things balanced. The first half of
the data is left on the old page, and the rest of the data is added to a new page — thus you have about a
50-50 split, and your tree remains balanced.
Figure 9-3
If you think about this splitting process a bit, you’ll realize that it adds a substantial amount of overhead
at the time of the split. Instead of inserting just one page, you are:
❑ Creating a new page
Chapter 9: SQL Server Storage and Index Structures
❑ Migrating rows from the existing page to the new page
❑ Adding your new row to one of the pages
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❑ Adding another entry in the parent node
Ordered insert as middle record
in a Cluster Key
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New record to be inserted,
but the page is full.
Since the new record needs to go in the
middle, the page must be split.
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