During a call to the built-in XML_SCHEMA_NAMESPACE() function on a large XML schema collection, or when you try to drop large XML schema collections, an out-of-memory condition may occur. The following are solutions you can use to handle this:
When the system load is light, use the DROP_XML_SCHEMA_COLLECTION command. If this fails, put the database in single-user mode by using the ALTER DATABASE statement and trying DROP XML SCHEMA COLLECTION again. If the XML schema collection exists in master, model, or tempdb, a server restart is required for single-user mode.
When you call the XML_SCHEMA_NAMESPACE, you can try to retrieve a single XML schema namespace, you can try the call when the system load is lighter, or you can try the call in single-user mode.