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The Filter Operator

Hugo Cornelis continues a dive into execution plan operators:

The Filter operator is used to remove rows from a result set that do not satisfy a specified condition.

Its most common usage is to implement a WHERE clause, although many elements of a WHERE clause get implemented in other ways (e.g. by pushing them down into an Index Seek or Index Scan operator), and Filter operators can also be used when the optimizer transforms other query elements into any type of filtering.

The Filter operator can only verify conditions based on the data in the current row; if data from different rows needs to be compared for the filter, the optimizer generates an execution plan that first adds the data to be compared to the row, or it uses different operators.

Click through for Hugo’s normal in-depth look.