Why Use SELECT TOP 100 Percent
Prior to SQL Server 2005, TOP 100 PERCENT was used in SELECT clauses to enable order by usage in view definitions. However, even in dynamic SQL statements in modern environments, this practice persists.
Intermediate Materialization
The primary reason for using TOP 100 PERCENT is for "intermediate materialization." As explained by Adam Machanic, this technique spools query results temporarily, allowing for more efficient subsequent processing. Specifically, it enables row-by-row processing of an intermediate result set, which can optimize performance in certain scenarios.
Example
Consider the following query:
SELECT foo
FROM
(SELECT foo From MyTable WHERE ISNUMERIC (foo) = 1) bar
WHERE
CAST(foo AS int) > 100
In SQL Server 2000, this query might fail due to unpredictable evaluation order. TOP 100 PERCENT can be used to enforce row-by-row processing, ensuring that the results are equivalent to those obtained by executing the queries sequentially:
SELECT foo From MyTable WHERE ISNUMERIC (foo) = 1 SELECT foo FROM bar WHERE CAST(foo AS int) > 100
This behavior continues to work in SQL Server 2005, despite the introduction of more sophisticated optimization techniques.
Precaution
While SELECT TOP 100 PERCENT can provide performance benefits in specific circumstances, it should be used judiciously. Changes to patch levels, schema, indexes, or row counts can inadvertently alter its impact on performance. Therefore, it's essential to fully understand the consequences before utilizing this technique.
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