Schema Caching (SQLXML 4.0)
Applies to: 
SQL Server
Azure SQL Database
With a side-by-side installation of XML for Microsoft SQL Server 2000 Web Release 1, Microsoft SQLXML 2.0, and SQLXML 3.0, you can explicitly control the schema caching in all versions by using the following registry keys:
Web Release 1:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\MSSQLServer\Client\SQLXMLX\SchemaCacheSize
SQLXML 2.0:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\MSSQLServer\Client\SQLXML2\SchemaCacheSize
SQLXML 3.0:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\MSSQLServer\Client\SQLXML3\SchemaCacheSize
Caution
Incorrectly editing the registry can severely damage your system. Before making changes to the registry, we recommend that you back up any valued data on the computer.
For more information about side-by-side installation, see What's New in SQLXML 4.0 SP1.
Schema caching significantly improves the performance of an XPath query. When an XPath query is executed against a mapping schema, the schema is stored in memory, and the necessary data structures are built in memory. If schema caching is set, the schema remains in memory, thereby improving performance for subsequent XPath queries.
You can set the schema cache size by adding the above key in the registry
The schema size is set based on the available memory and the number of schemas you are using. The default SchemaCacheSize size is 31. If you set SchemaCacheSize higher, more memory is used. Therefore, you can increase the cache size if schema access seems slow, or decrease the cache size if memory is low.
For performance reasons, it is recommended that you set SchemaCacheSize higher than the number of mapping schemas you usually use. As the number of schemas increase, if SchemaCacheSize is less than the number of schemas you have, the performance degrades.
Note
During development, it is recommended that you do not cache the schemas, because the changes to the schemas are not reflected in the cache for about two minutes.