强制SQL Server首先评估子查询

Question

How can I force SQL Server to Evaluate a sub query first?

My query looks something like:

SELECT ObjectId FROM
(SELECT ObjectId FROM Table Where Id = @Id) T
WHERE fn_LongRunningFunction(T.ObjectId) = 1

I want the outer where clause to evaluate on the result of the inner query. How would I do this without inserting the sub query into a temp table.

When I execute this query SQL evaluates the query as if it where written like:

SELECT ObjectId FROM Table Where Id = @Id AND fn_LongRunningFunction(ObjectId) = 1

which is not what I want.

Answer 1

Why you're even using a sub-query here you could have simply used one query here something like ....

SELECT ObjectId 
FROM Table 
Where Id = @Id
AND fn_LongRunningFunction(ObjectId) = 1

Note

Using a scalar function in where clause as you have will cause a full table scan, Since sql server has to touch every row in the column and execute the function on ObjectId column values to evaluate if it is equal to 1 or not.

Avoid using any functions in where clauses on column names whenever possible.

for example if you are evaluating a value of a column against a given value do the reverse on the other side of the comparison operator and leave the column alone in where clause, for example if you are looking for values in a table

WHERE ColumnName + 20 < 100 

Instead of doing you could do something like

WHERE ColumnName < 100 -20 

In first example sql server will have to touch every row and will add 20 to its value to evaluate it against 100 which will cause a table scan.

In 2nd example if sql server has an index on that column it will simply do a seek to see which values are less then 100 -20.