How Does SELECT TOP 100 PERCENT Enable Intermediate Materialization in SQL Server?

Intermediate Materialization with SELECT TOP 100 Percent

Before SQL Server 2005, a technique called "intermediate materialization" could be employed to force SQL Server to honor an ORDER BY clause within a view definition by including TOP 100 PERCENT in the SELECT statement.

However, its use has extended beyond view definitions in dynamic SQL statements. The primary purpose of SELECT TOP 100 PERCENT in this context is to leverage intermediate materialization. This technique involves creating an intermediate result set by placing the ORDER BY clause within the TOP 100 PERCENT construct.

One example where this technique may be beneficial is when you need to apply a filter on the results of a nested query. Consider the following query:

SELECT foo
FROM (SELECT foo FROM MyTable WHERE ISNUMERIC (foo) = 1) bar
WHERE CAST(foo AS int) > 100

In this example, the inner query returns a result set that is then filtered in the outer query. However, the ORDER BY clause in the inner query may not be honored, leading to unpredictable results.

By adding SELECT TOP 100 PERCENT to the inner query, you can force intermediate materialization:

SELECT foo
FROM (SELECT TOP 100 PERCENT foo FROM MyTable WHERE ISNUMERIC (foo) = 1 ORDER BY foo) bar
WHERE CAST(foo AS int) > 100

This intermediate materialization ensures that the result set from the inner query is ordered before being filtered in the outer query, providing the desired outcome.

It's important to note that this technique should only be used when necessary and with caution, as changes in server configurations (e.g., patch level, schema, index, row counts) could potentially disrupt its functionality.

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