Blog Post

Spools in Execution Plans


I got the question the other day, when are you likely to see a spool in an execution plan? Easy, whenever SQL Server needs to walk through the data multiple times, usually in JOIN operations… Yeah, well, once again, my flip answers are not quite the entire story.

Spool operations are temporary storage of the data for later reuse in a query plan. There are two types of spool operations, eager spool and lazy spool. A spool is basically a temporary table created within the execution of the query that is used when it’s likely that data will be needed again, and again during the execution of the query. This is not an explicit #temp temporary table, but a work table for operations within the processing necessary for a given query’s behavior. A spool is created when the optimizer thinks that it can work better with a semi-permanent sub-set of data rather than have to perform multiple seeks or scans against a table or index or in other places where data re-use is important (more in a bit).

So how does this work? Take a look at a simple query:

UPDATE Person.Person
SET FirstName = 'Ted'
WHERE FirstName = 'Ted';

When the execution plan for this query is generated, it looks like this:


In this case, an eager spool is used as part of the roll back mechanism and to prevent the Halloween scenario. An eager spool is one where the data is retrieved immediately.

It’s possible to see the other type of spool in a query that looks like this (straight out of the Books Online):

WITH DirectReports(ManagerID, EmployeeID, EmployeeLevel) AS
    SELECT ManagerID, EmployeeID, 0 AS EmployeeLevel
    FROM HumanResources.Employee
    SELECT e.ManagerID, e.EmployeeID, EmployeeLevel + 1
    FROM HumanResources.Employee e
        INNER JOIN DirectReports d
        ON e.ManagerID = d.EmployeeID
SELECT ManagerID, EmployeeID, EmployeeLevel
FROM DirectReports ;

Which would result in this execution plan:


Now you see a table spool that is called a lazy spool. This means that it only loads data as the data is requested. This makes a lot of sense because the lazy spool is operating as the means for gathering the recursive data together. So it’s not going to go and get all the data available, like an eager spool. Instead it’s going to only load the data as needed, lazy.

These two scenarios are much more likely than the typical join to show a table spool. Yes, it can, and does, appear in join operations, but as I said at the beginning, that’s such a flip answer. Much better to try to be complete.


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