Hugo Kornelis continues a series on storage structures:
After covering on-disk rowstore, columnstore indexes, memory-optimized storage, and memory-optimized columnstores, it is now time to turn our attention to storage structures that are used for specific datatypes only. The first “victim” will be the XML index.
When you need to store XML data in a SQL Server database, you can choose between two data types, each with their pros and cons. You can choose to store the XML data as nvarchar(max). This preserves the exact content of the XML as you received it, which might be required for legal reasons. However, any query that cares about the specific content of the XML data will have to resort to very complicated string expressions.
The xml data type shreds the XML and stores the content in an internal format, that allows SQL Server to work with for instance XQuery or XPath expressions. This format also saves space as compared to the nvarchar(max) alternative. However, when you query the data, the content will be the same, but formatting and whitespace might be different.
The latter also allow you to build indexes on top of them, and those indexes are what Hugo covers.