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

works only if tempdb is on a separate physical drive from your database file; otherwise, the change is
only in name, and the competition for I/O is still a factor.
ONLINE
If you’re going to use SORT_IN_TEMPDB, make sure that there is enough space in tempdb for large
files.
Setting this to ON forces the table to remain available for general access, and does not create any locks
that block users from the index and/or table. By default, the full index operation will grab the locks
(eventually a table lock) needed for full and efficient access to the table. The side effect, however, is that
your users are blocked out. (Yeah, it’s a paradox; you’re likely building an index to make the database
more usable, but you essentially make the table unusable while you do it.)
Now, you’re probably thinking something like: “Oh, that sounds like a good idea — I’ll do that every
time so my users are unaffected.” Poor thinking. Keep in mind that any index construction like that is
probably a very I/O-intensive operation, so it is affecting your users one way or the other. Now, add that
there is a lot of additional overhead required in the index build for it to make sure that it doesn’t step on
the toes of any of your users. If you let SQL Server have free reign over the table while it’s building the
index, then the index will be built much faster, and the overall time that the build is affecting your system
will be much smaller.
ONLINE index operations are supported only in the Enterprise Edition of SQL Server. You can execute
the index command with the ONLINE directive in other editions, but it will be ignored, so don’t be surprised
if you use ONLINE and find your users still being blocked out by the index operation if you’re
using a lesser edition of SQL Server.
ALLOW ROW/PAGE LOCKS
This is a longer term directive than ONLINE and is a very, very advanced topic. For purposes of this book
and taking into consideration how much we’ve introduced so far on locking, let’s stick with a pretty simple
explanation.
Through much of the book thus far, we have repeatedly used the term “lock.” As explained early on,
this is something of a placeholder to avoid conflicts in data integrity. The ALLOW settings we’re looking at
here determine whether this index will allow row or page locks or not. This falls under the heading of
extreme performance tweak.
MAXDOP
Chapter 9: SQL Server Storage and Index Structures
This is overriding the system setting for the maximum degree of parallelism for purposes of building
this index. Parallelism is not something I talk about elsewhere in this book, so we’ll give you a mini-dose
of it here.
In short, the degree of parallelism is how many processes are put to use for one database operation (in
this case, the construction of an index). There is a system setting called the max degree of parallelism
that allows you to set a limit on how many processors per operation. The MAXDOP option in the index
creation options allows you to set the degree of parallelism to be either higher or lower than the base
system setting, as you deem appropriate.
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