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

Secondary XML Indexes
Chapter 9: SQL Server Storage and Index Structures
Nothing special here — much as non-clustered indexes point to the cluster key of the clustered index,
secondary XML indexes point at primary XML indexes. Once you create a primary XML index, you can
create up to 248 more XML indexes on that XML column.
Implied Indexes Created with Constraints
I guess I call this one “index by accident.” It’s not that the index shouldn’t be there — it has to be there if
you want the constraint that created the index. It’s just that I’ve seen an awful lot of situations where the
only indexes on the system were those created in this fashion. Usually, this implies that the administrators
and/or designers of the system are virtually oblivious to the concept of indexes.
However, you’ll also find yet another bizarre twist on this one — the situation where the administrator
or designer knows how to create indexes, but doesn’t really know how to tell what indexes are already on
the system and what they are doing. This kind of situation is typified by duplicate indexes. As long as
they have different names, SQL Server will be more than happy to create them for you.
Implied indexes are created when one of two constraints is added to a table:
❑ A PRIMARY KEY
❑ A UNIQUE constraint (aka, an alternate key)
We’ve seen plenty of the CREATE syntax up to this point, so I won’t belabor it; however, it should be
noted that all the options except for {CLUSTERED|NONCLUSTERED} and FILLFACTOR are not allowed
when creating an index as an implied index to a constraint.
Creating Indexes on Sparse and Geospatial Columns
These are definitely beyond the scope of a beginning book, but sparse columns and data with a geospatial
data type can both have special indexes created on them. Again, keep this in mind if you have application
for these special-needs types of columns.
Choosing Wisely: Deciding What Index
Goes Where and When
By now, you’re probably thinking to yourself, “Gee, I’m always going to create clustered indexes!” There
are plenty of good reasons to think that way. Just keep in mind that there are also some reasons not to.
Choosing what indexes to include and what not to can be a tough process, and in case that wasn’t
enough, you have to make some decisions about what type you want them to be. The latter decision is
made simultaneously easier and harder by the fact that you can only have one clustered index. It means
that you have to choose wisely to get the most out of it.
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