Measuring Query Performance : “Execution Plan Query Cost” vs “Time Taken”

Question

I\'m trying to determine the relative performance of two different queries and have two ways of measuring this available to me:
1. Run both and time each query
2. Ru

Answer 1

I understand it’s an old question – however I would like to add an example where cost is same but one query is better than the other.

As you observed in the question, % shown in execution plan is not the only yardstick for determining best query. In the following example, I have two queries doing the same task. Execution Plan shows both are equally good (50% each). Now I executed the queries with SET STATISTICS IO ON which shows clear differences.

In the following example, the query 1 uses seek whereas Query 2 uses scan on the table LWManifestOrderLineItems. When we actually checks the execution time however it is find that Query 2 works better.

Also read When is a Seek not a Seek? by Paul White

QUERY

---Preparation---------------
-----------------------------
DBCC FREEPROCCACHE
GO
DBCC DROPCLEANBUFFERS
GO

SET STATISTICS IO ON  --IO
SET STATISTICS TIME ON

--------Queries---------------
------------------------------

SELECT LW.Manifest,LW.OrderID,COUNT(DISTINCT LineItemID)
FROM LWManifestOrderLineItems LW
INNER JOIN ManifestContainers MC
    ON MC.Manifest = LW.Manifest
GROUP BY LW.Manifest,LW.OrderID
ORDER BY COUNT(DISTINCT LineItemID) DESC  

SELECT LW.Manifest,LW.OrderID,COUNT( LineItemID) LineCount
FROM LWManifestOrderLineItems LW
WHERE LW.Manifest IN (SELECT Manifest FROM ManifestContainers)
GROUP BY LW.Manifest,LW.OrderID
ORDER BY COUNT( LineItemID) DESC  

Statistics IO

Execution Plan