使用CROSS APPLY的查询速度慢

Question

I have the following SQL code which takes several seconds to execute:

SELECT instance.startdate
      ,instance.enddate 
FROM (select mr.id 
      FROM meeting_recurrence AS mr 
      inner JOIN meeting_details md ON md.id = mr.meeting_template_id 
      WHERE host_user_id=17) AS p 
CROSS APPLY GetRecurrenceMeetingInstances(p.id,'2018-11-09 11:00:00','2018-11-09 15:00:00') AS instance  
WHERE 21015 IS NOT NULL 
AND instance.meetingid <> 21015

The internal select (select mr.id FROM meeting_recurrence...) returns in the above example instantly, with two ids.

Running GetRecurrenceMeetingInstances() with those two ids separately, returns empty results instantly.

Why does running the entire statement take so long compared to running them separately?

Execution plan image: https://imgur.com/a/3Ydym

Answer 1

It seems CROSS APPLY was calling GetRecurrenceMeetingInstances() a lot more than i thought. I solved the problem by saving the inner select in a local variable first:

DECLARE @innerresult table(meetingid int)
INSERT INTO @innerresult(meetingid) (select mr.id FROM meeting_recurrence AS mr inner JOIN meeting_details md ON md.id = mr.meeting_template_id 
    WHERE host_user_id=17)

INSERT INTO @timeslots (startdate,enddate) 
    SELECT instance.startdate,instance.enddate 
    FROM (select meetingid FROM @innerresult) AS p 
    CROSS APPLY GetRecurrenceMeetingInstances(p.meetingid,'2018-11-09 11:00:00','2018-11-09 15:00:00') AS instance