SQL Server Execution Plans - Red Gate Software

UPDATE statements
Chapter 2: Graphical Execution Plans for Basic Queries
UPDATE statements are also against one table at a time. Depending on the structure of the
table, and the values to be updated, the impact on the execution plan could be as severe as
that shown above for the INSERT query. Consider the UPDATE statement in Listing 2.16.
UPDATE [Person].[Address]
SET [City] = 'Munro',
[ModifiedDate] = GETDATE()
WHERE [City] = 'Monroe' ;
Listing 2.16
Figure 2.36 shows the estimated execution plan.
Figure 2.36
Let's begin reading this execution plan in the physical operation order, from right to
left. The first operator is a NonClustered Index Scan, which retrieves all of the necessary
rows from a non-clustered index, scanning through them, one row at a time. This is
not particularly efficient and should be a flag to you that perhaps the table needs better
indexes to speed performance. The purpose of this operator is to identify all the rows
WHERE [City] = 'Monroe', and then send them to the next operator.
The next operator is Top. In an execution plan for an UPDATE statement, the Top
operator enforces row count limits, if there are any. In this case, no limits were enforced
because the TOP clause was not used in the UPDATE query as you can see in Listing 2.16.
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