Optimize deduplication join in UpdateRelationships

In a previous commit, I introduced the ability to prevent `UpdateRelationships` from trying to insert duplicate `Target_TopicID`s. This isn't currently necessary since `SqlTopicRepository` always deletes the relationships (via `UpdateTopic`'s `@DeleteRelationships` flag) before calling `UpdateRelationships`. Nevertheless, in the case that this wasn't done, this avoids a primary key constraint violation if existing keys are sent via `@RelatedTopics`.

Unfortunately, the process by which this was done has proven very expensive, due to the performance of user-defined, table-valued types. While those work fine for data transfer objects _to_ stored procedures, they are suboptimal for internal storage _within_ stored procedures. In this case, it's also completely unnecessary.

To mitigate this, the local `@Existing_TopicIDs` storage is replaced with a new `LEFT JOIN` against the _actual_ `Relationships` table. This proves dramatically faster in testing (representing ≈12% of the test case's execution plan, as opposed to ≈38% with the previous model). Note that the test case includes inserting sample data into the `@RelatedTopics` parameter, so that ends up taking up _most_ of the time.

Author: JeremyCaney

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

@@ -10,23 +10,6 @@ CREATE PROCEDURE [dbo].[UpdateRelationships]
 	@RelatedTopics		TopicList	READONLY
 AS
 
---------------------------------------------------------------------------------------------------------------------------------
--- DECLARE AND SET VARIABLES
-
-DECLARE	@Existing_TopicIDs	TopicList
-
---------------------------------------------------------------------------------------------------------------------------------
--- IDENTIFY EXISTING VALUES
---------------------------------------------------------------------------------------------------------------------------------
-INSERT
-INTO	@Existing_TopicIDs (
-	  TopicID
-	)
-SELECT	Target_TopicID
-FROM	Relationships
-WHERE	Source_TopicID		= @TopicID
-  AND	RelationshipKey		= @RelationshipKey
-
 --------------------------------------------------------------------------------------------------------------------------------
 -- INSERT NOVEL VALUES
 --------------------------------------------------------------------------------------------------------------------------------
@@ -38,11 +21,12 @@ INTO	Relationships (
 	)
 SELECT	@TopicId,
 	@RelationshipKey,
-	Target.TopicID
+	TopicID
 FROM	@RelatedTopics		Target
-FULL JOIN	@Existing_TopicIDs	Existing
-  ON	Existing.TopicID	= Target.TopicID
-WHERE	Existing.TopicID	is null
+LEFT JOIN	Relationships		Existing
+  ON	Target_TopicID		= TopicId
+  AND	Source_TopicID		= @TopicID
+WHERE	Target_TopicID		IS NULL
 
 --------------------------------------------------------------------------------------------------------------------------------
 -- RETURN TOPIC ID