SQL Server Execution Plans - Red Gate Software

Chapter 6: Cursor Operations
The major difference is evident in how the Fetch Query works, in order to support the
updating of data after the cursor was built. Figure 6.20 shows that portion of the plan in
more detail.
Figure 6.20
Going to the right and top of the Fetch Query definition, we find that it first retrieves the
key from the index created in the Population Query. Then, to retrieve the data, it joins it
through a Nested Loop operation to the Sales.Currency table. This is how the Keyset
cursor manages to get updated data into the set returned while the cursor is active.
The Constant Scan operator scans an internal table of constants; basically, it's just a
place for adding data later in the plan. The data from the Constant Scan feeds into the
Clustered Index Update operator in order to be able to change the data stored inside
the cursor, if necessary. This data is joined to the first set of data through a Nested Loop
operation and finishes with a Compute Scalar, which represents the row number.
Physical operators
When the cursor is executed, we get the plan shown in Figure 6.21.
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