Can UNION ALL be faster than JOINs or do my JOINs just suck?

Question

I have a Notes table with a uniqueidentifier column that I use as a FK for a variety of other tables in the database (don\'t worry, the uniqu

Answer 1

UNION is slower, but UNION ALL should be pretty quick, right?

Answer 2

I may be wrong, but I think that you will get a better performance if you rewrite you JOIN version to

SELECT N.*  
FROM Notes N  
LEFT JOIN Leads L ON N.TargetUniqueID = L.UniqueID AND L.LeadID = @LeadID  
LEFT JOIN Quotes Q ON N.TargetUniqueID = Q.UniqueID  AND Q.LeadID = @LeadID
WHERE Q.LeadID IS NOT NULL OR L.LeadID IS NOT NULL

Answer 3

The UNION ALL version would probably be satisfied quite easily by 2 index seeks. OR can lead to scans. What do the execution plans look like?

Also have you tried this to avoid accessing Notes twice?

;WITH J AS
(
SELECT UniqueID FROM Leads WHERE LeadID = @LeadID
UNION ALL
SELECT UniqueID FROM Quotes WHERE LeadID = @LeadID
)

SELECT N.*  /*Don't use * though!*/
FROM Notes N  
JOIN J ON N.TargetUniqueID = J.UniqueID  

Answer 4

In my experience, SQL Server is really bad with join conditions containing OR. I also use UNIONs in that case, and I got similar results like you (maybe half a second instead of 20).

Who said UNIONS are bad? Especially if you use UNION ALL, there should not be a performance hit, as UNION would have to go through the result to only keep unique records (actually doing something like distinct or group by).

Answer 5

You second query wouldn' even give correct results as it would covert the left joins to inner joins, see here for explantion as to why your syntax is bad:

http://wiki.lessthandot.com/index.php/WHERE_conditions_on_a_LEFT_JOIN