How do I force SQL Server to execute a query in a particular order

Question

I have the following query

    
DECLARE @userId INT  
DECLARE @siteId INT

SET @siteId = -1  
SET @userId = 1828  

SELECT  a.id AS alertId,  
        a.location_id,  
        a.alert_type_id,  
        a.event_id,  
        a.user_id,  
        a.site_id,  
        a.accepted_by  
FROM    alerts AS a    
JOIN    alert_types AS ats ON a.alert_type_id = ats.id 
JOIN    events AS tr ON a.event_id = tr.event_id 
WHERE   tr.end_Time IS null
AND     tr.status_id = 0
AND     ats.code = 'E'
AND     a.site_id in (SELECT * FROM dbo.udf_get_event_sitelist(@siteId, @userId))

This query takes between 5 and 17 seconds to run, however under many circumstances the function dbo.udf_get_event_sitelist(@siteId, @userId) returns no rows, so the query will not find any data.

How can I force SQL Server to execute the user defined function first. I appreciate that I could rewrite the query into a stored procedure and perform the sub-select first, however I would like to do it in a single SQL statement if possible.

Answer 1

make the "FROM" table the results set of the function and join the other tables to it

DECLARE @userId INT  
DECLARE @siteId INT

SET @siteId = -1  
SET @userId = 1828  

SELECT  a.id AS alertId,  
        a.location_id,  
        a.alert_type_id,  
        a.event_id,  
        a.user_id,  
        a.site_id,  
        a.accepted_by  
FROM    (SELECT * FROM dbo.udf_get_event_sitelist(@siteId, @userId)) dt
JOIN    alerts AS a ON dt.site_id=a.site_id
JOIN    alert_types AS ats ON a.alert_type_id = ats.id 
JOIN    events AS tr ON a.event_id = tr.event_id 
WHERE   tr.end_Time IS null
AND     tr.status_id = 0
AND     ats.code = 'E'

Answer 2

you could select the results of udf_get_event_sitelist into a table variable and only proceed with the big query if @@rowcount > 0

Answer 3

The problem that you have when using inline functions is that they can be re-evaluated for each row returned in the SELECT. This means, that if the SELECT statement returns 100 rows, then the function can be executed 100 times.

You should really follow Sambo99's advice and extract it to a table variable (or a temp table if you think it needs indexes).

Answer 4

Also, modify you UDF to only return the site_ID as I am guessing you do not neet all (*) columns

SELECT * FROM dbo.udf_get_event_sitelist(@siteId, @userId)

to

SELECT site_id FROM dbo.udf_get_event_sitelist(@siteId, @userId)