Lookup @PreviousExtendedAttributes from table directly

While the `ExtendedAttributeIndex` is quite useful, it also introduces significant performance overhead. In the case of `UpdateTopic`, it represents 99% of the actual execution plan when it comes to just the `SELECT` statements. (Obviously, the `INSERT` statements are comparatively more expensive.)

This is a pretty steep price given that it's not really necessary here. When we're working with a single topic, and only need a single record, we can grab the last extended attribute directly from the `ExtendedAttributes` with only a couple of additional lines of code (namely a `TOP 1` and a `ORDER BY` clause).

This greatly improves the performance of `UpdateTopic`, and distributes the weight across other tasks.

Author: JeremyCaney

OnTopic.Data.Sql.Database/Stored Procedures/UpdateTopic.sql

@@ -41,9 +41,11 @@ WHERE	AttributeKey		!= 'ParentId'
 --------------------------------------------------------------------------------------------------------------------------------
 DECLARE	@PreviousExtendedAttributes	XML
 
-SELECT	@PreviousExtendedAttributes	= AttributesXml
-FROM	ExtendedAttributeIndex
+SELECT	TOP 1
+	@PreviousExtendedAttributes	= AttributesXml
+FROM	ExtendedAttributes
 WHERE	TopicID		= @TopicID
+ORDER BY	Version		DESC
 
 --------------------------------------------------------------------------------------------------------------------------------
 -- ADD EXTENDED ATTRIBUTES, IF CHANGED