I have to say, I only recently noticed this on a tool tip:

and this in the property sheet:

The bad news is, I noticed them while presenting. The worse news is, I said them out loud and then, inevitably, someone in the audience, I forget who it was, might have been Neil Hambly (blog|twitter) said, “What’s that?” Which left me standing there with a slack-jawed expression (one that comes naturally from years & years of practice). I didn’t know. I couldn’t remember having seen one before.

Here’s a query that you can run in AdventureWorks2008R2 to get a look at this critter:

SELECT  soh.PurchaseOrderNumber,
soh.AccountNumber,
p.Name,
sod.OrderQty,
sod.LineTotal,
cc.CardNumber
FROM    Sales.SalesOrderHeader AS soh
JOIN Sales.SalesOrderDetail AS sod
ON soh.SalesOrderID = sod.SalesOrderID
JOIN Production.Product AS p
ON sod.ProductID = p.ProductID
JOIN Sales.CreditCard AS cc
ON soh.CreditCardID = cc.CreditCardID;

The entire execution plan looks like this:

We’re focused on the Hash Match join at the top. For logical processing it would be the first operation. For physical operations it would be the last.

The way a Hash Match works is by creating a hash table (in tempdb by the way) and making a hash value. Then, it makes a hash value of the stuff being compared and tries to find matches in the hash table. That’s it.

The residual is if there are additional predicates that also must be matched in order to fully satisfy the query. That’s all. It’s actually quite simple.

What are the implications? Well, this is where it gets fun. You see, the first match, in the hash has to take place, and then, it also has to do the residual probe. The first match is part of the process. The second match is additional work. That’s not good. You can see it in this example plan because the Hash Match operation is estimated as the most costly and that’s probably true.

The key is, drill down to understand what your execution plans are up to.