Question

This query takes 16 seconds to run

SELECT 
    WO.orderid
FROM 
    WebOrder as WO
    INNER JOIN Addresses AS A ON WO.AddressID = A.AddressID
    LEFT JOIN SalesOrders as SO on SO.SO_Number = WO.SalesOrderID   

If I comment out either of the joins, it runs in a small fraction of a second. Example:

SELECT 
    WO.orderid
FROM 
    WebOrder as WO
    INNER JOIN Addresses AS A ON WO.AddressID = A.AddressID
    -- LEFT JOIN SalesOrders as SO on SO.SO_Number = WO.SalesOrderID    

or

SELECT 
    WO.orderid
FROM 
    WebOrder as WO
    -- INNER JOIN Addresses AS A ON WO.AddressID = A.AddressID
    LEFT JOIN SalesOrders as SO on SO.SO_Number = WO.SalesOrderID

Notes

• There exists about 40,000 records each in tables SalesOrders and Adddresses.
• I have indexes or PKeys on all fields used in the ON clauses.

Execution Plan for the slow version (SalesOrders Join commented out)

Execution Plan for fast version

Why do these joins when used in conjunction with one another cause this to go from ~0.01 seconds to 16 seconds?

Answer 1

Your execution plan doesn’t show any expensive operations, I would try to following to troubleshoot bad performance:

• Rebuild Indexes
• Update Stats
• DBCC FREEPROCCACHE

Personally I wouldn’t expect the latter to do anything — it looks like you have a sensible query plan as it is.