Merge branch 'feature/UpdateTopic-performance' into develop

While the `feature/track-dirty-relationships` (afa1e9b) greatly improved the performance of recursively saving a large topic hierarchy by avoiding unnecessary and expensive calls to the `UpdateTopic` stored procedure, it did nothing to make that call itself less expensive. This update fixes that by reviewing and optimizing the `UpdateTopic` stored procedure, while simultaneously including additional functionality. In testing, the stored procedure is about twice as fast as it was previously, taking 18 seconds to update ≈250 topics, instead of the previous 38 seconds. And that's _with_ a new subquery to ensure that none of the indexed attributes are duplicating their previous value—something we currently do in `SqlTopicRepository`, but which is preferably enforced by the stored procedure itself.

This update includes:
- Lookup `@PreviousExtendedAttributes` directly from `ExtendedAttributes` instead of going through `ExtendedAttributeIndex` (b76ac69)
- Explored various performance improvements to nested query for null attributes, but found the current query to be optimal (4035bcc)
- Applied nested query to indexed attributes handling in order to prevent duplicates from being created (78dd6cd)

Author: JeremyCaney

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

@@ -32,18 +32,29 @@ SELECT	@TopicID,
 	AttributeKey,
 	AttributeValue,
 	@Version
-FROM	@Attributes
+FROM	@Attributes		New
+OUTER APPLY (
+    SELECT	TOP 1
+	AttributeValue		AS ExistingValue
+    FROM	Attributes
+    WHERE	TopicID		= @TopicID
+      AND	AttributeKey		= New.AttributeKey
+    ORDER BY	Version		DESC
+  )			Existing
 WHERE	AttributeKey		!= 'ParentId'
-  AND	IsNull(AttributeValue, '')	!= ''
+  AND	ISNULL(AttributeValue, '')	!= ''
+  AND 	ISNULL(ExistingValue, '')	!= AttributeValue
 
 --------------------------------------------------------------------------------------------------------------------------------
 -- PULL PREVIOUS EXTENDED ATTRIBUTES
 --------------------------------------------------------------------------------------------------------------------------------
 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
@@ -76,16 +87,17 @@ SELECT	@TopicID,
 	AttributeKey,
 	'',
 	@Version
-FROM	@Attributes		NullAttributes
+FROM	@Attributes		New
+CROSS APPLY (
+  SELECT	TOP 1
+	AttributeValue		AS ExistingValue
+  FROM	Attributes
+  WHERE	TopicID		= @TopicID
+    AND	AttributeKey		= New.AttributeKey
+  ORDER BY	Version DESC
+)			Existing
 WHERE	IsNull(AttributeValue, '')	= ''
-  AND (
-    SELECT	TOP 1
-	AttributeValue
-    FROM	Attributes
-    WHERE	TopicID		= @TopicID
-      AND	AttributeKey		= NullAttributes.AttributeKey
-    ORDER BY	Version DESC
-  )			!= ''
+  AND	ExistingValue		!= ''
 
 --------------------------------------------------------------------------------------------------------------------------------
 -- REMOVE EXISTING RELATIONS