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

occur, and that one or more non-leaf level page must also be modified to have the reference to the proper
leaf page.
Sometimes — quite often actually — not creating that extra index is the thing to do. Sometimes, the best
thing to do is choose your indexes based on the transactions that are critical to your system and use the
table in question. Does the code for the transaction have a WHERE clause in it? What column(s) does it
use? Is there a sorting required?
Choosing That Clustered Index
Remember that you can have only one, so you need to choose it wisely.
By default, your primary key is created with a clustered index. This is often the best place to have it, but
not always (indeed, it can seriously hurt you in some situations), and if you leave things this way, you
won’t be able to use a clustered index anywhere else. The point here is don’t just accept the default.
Think about it when you are defining your primary key — do you really want it to be a clustered index?
If you decide that you indeed want to change things, that is, you don’t want to declare things as being
clustered, just add the NONCLUSTERED keyword when you create your table. For example:
CREATE TABLE MyTableKeyExample
(
Column1 int IDENTITY
PRIMARY KEY NONCLUSTERED,
Column2 int
)
Chapter 9: SQL Server Storage and Index Structures
Once the index is created, the only way to change it is to drop and rebuild it, so you want to get it set
correctly up front.
Keep in mind that if you change which column(s) your clustered index is on, SQL Server will need to do
a complete resorting of your entire table (remember, for a clustered index, the table sort order and the
index order are the same). Now, consider a table you have that is 5,000 characters wide and has a million
rows in it — that is an awful lot of data that has to be reordered. Several questions should come to mind
from this:
❑ How long will it take? It could be a long time, and there really isn’t a good way to estimate that
time.
❑ Do I have enough space? Figure that in order to do a resort on a clustered index, you will, on
average, need an additional 1.2 times (the working space plus the new index) the amount of
space your table is already taking up. This can turn out to be a very significant amount of space
if you’re dealing with a large table — make sure you have the room to do it in. All this activity
will, by the way, happen in the database itself, so this will also be affected by how you have
your maximum size and growth options set for your database.
❑ Should I use the SORT_IN_TEMPDB option? If tempdb is on a separate physical array from your
main database and it has enough room, then the answer is probably yes.
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