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

Chapter 9: SQL Server Storage and Index Structures
ON
SQL Server gives you the option of storing your indexes separately from the data by using the ON option.
This can be nice from a couple of perspectives:
❑ The space that is required for the indexes can be spread across other drives.
❑ The I/O for index operations does not burden the physical data retrieval.
There’s more to this, but this is hitting the area of highly advanced stuff. It is very data and use dependent,
and so we’ll consider it out of the scope of this book.
Creating XML Indexes
XML indexes were first added in SQL Server 2005. The indexing of something as unstructured as XML
has been a problem that many have tried to solve, but few have done with any real success, so count
your lucky stars that you have this fine tool available if you’re storing XML data. Let’s get to what XML
indexes are about.
This is another of those “chicken or egg?” things, in that we haven’t really looked at XML at all in this
book thus far. Still, I consider this more of an index topic than an XML topic. Indeed, the XML create
syntax supports all the same options we saw in the previous look at the CREATE statement, with the
exception of IGNORE_DUP_KEY and ONLINE. So, for a bit of hyper-fast background:
Unlike the relational data that we’ve been looking at thus far, XML tends to be relatively unstructured.
It utilizes tags to identify data, and can be associated with what’s called a schema to provide type and
validation information to that XML-based data. The unstructured nature of XML requires the notion of
“navigating” or “path” information to find a data “node” in an XML document. Now indexes, on the
other hand, try and provide very specific structure and order to data — this poses something of a
conflict.
You can create indexes on columns in SQL Server that are of XML type. The primary requirements of
doing this are:
❑ The table containing the XML you want to index must have a clustered index on it.
❑ A “primary” XML index must exist on the XML data column before you can create “secondary”
indexes (more on this in a moment).
❑ XML indexes can be created only on columns of XML type (and an XML index is the only kind
of index you can create on columns of that type).
❑ The XML column must be part of a base table — you cannot create the index on a view.
The Primary XML Index
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The first index you create on an XML index must be declared as a “primary” index. When you create a
primary index, SQL Server creates a new clustered index that combines the clustered index of the base
table together with data from whatever XML node you specify.