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

Chapter 9: SQL Server Storage and Index Structures
But the overhead doesn’t stop there. Since we’re in a tree arrangement, you have the possibility for
something of a cascading action. When you create the new page (because of the split), you need to make
another entry in the parent node. This entry in the parent node also has the potential to cause a page
split at that level, and the process starts all over again. Indeed, this possibility extends all the way up to
and can even affect the root node.
If the root node splits, then you actually end up creating two additional pages. Because there can be only
one root node, the page that was formerly the root node is split into two pages and becomes a new intermediate
level of the tree. An entirely new root node is then created, and will have two entries (one to the
old root node, one to the split page).
Needless to say, page splits can have a very negative impact on system performance and are characterized
by behavior where your process on the server seems to just pause for a few seconds (while the
pages are being split and rewritten).
We will talk about page-split prevention before we’re done with this chapter.
While page splits at the leaf level are a common fact of life, page splits at intermediate nodes happen far
less frequently. As your table grows, every layer of the index will experience page splits, but because the
intermediate nodes have only one entry for several entries on the next lower node, the number of page
splits gets less and less frequent as you move further up the tree. Still, for a split to occur above the leaf
level, there must have already been a split at the next lowest level — this means that page splits up the
tree are cumulative (and expensive performance-wise) in nature.
SQL Server has a number of different types of indexes (which we will discuss shortly), but they all make
use of this B-Tree approach in some way or another. Indeed, they are all very similar in structure, thanks
to the flexible nature of a B-Tree. Still, we shall see that there are indeed some significant differences, and
these can have an impact on the performance of our system.
For an SQL Server index, the nodes of the tree come in the form of pages, but you can actually apply this
concept of a root node, the non-leaf level, the leaf level, and the tree structure to more than just SQL
Server or even just databases.
How Data Is Accessed in SQL Server
In the broadest sense, there are only two ways in which SQL Server retrieves the data you request:
❑ Using a table scan
❑ Using an index
Which method SQL Server uses to run your particular query will depend on what indexes are available,
what columns you are asking about, what kind of joins you are doing, and the size of your tables.
Use of Table Scans
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A table scan is a pretty straightforward process. When a table scan is performed, SQL Server starts at the
physical beginning of the table, looking through every row in the table. As it finds rows that match the
criteria of your query, it includes them in the result set.