LEFT JOIN Significantly faster than INNER JOIN

Question

I have a table (MainTable) with a bit over 600,000 records. It joins onto itself via a 2nd table (JoinTable) in a parent/child type relationship:

Answer 1

The Left join seems to be faster because SQL is forced to do the smaller select first and then join to this smaller set of records. For some reason the optimiser doesn't want to do this naturally.

3 ways to force the joins to happen in the right order:

1. Select the first subset of data into a temporary table (or table variable) then join on it
2. Use left joins (and remember that this could return different data because it's a left join not an inner join)
3. use the FORCE ORDER keyword. Note that if table sizes or schemas change then the query plan may not be correct (see https://dba.stackexchange.com/questions/45388/forcing-join-order)

Answer 2

Try this one. Same result, different approach:

SELECT c.ID, p.ID 
FROM
(SELECT   Child.ID, JoinTable.ParentID
FROM     MainTable
AS       Child
JOIN     JoinTable
      ON Child.ID = JoinTable.ID) AS c
INNER JOIN 
(SELECT   Parent.ID, JoinTable.ID
FROM     MainTable
AS       Parent
JOIN     JoinTable
      ON Parent.ID = JoinTable.ParentID
     AND Parent.SomeOtherData = Child.SomeOtherData) AS p
ON c.ParentID = p.ID

If it does not help, use cte:

;WITH cte AS
(SELECT   Child.ID, JoinTable.ParentID
FROM     MainTable
AS       Child
JOIN     JoinTable
      ON Child.ID = JoinTable.ID)
SELECT cte.ID, Parent.ID
FROM cte INNER JOIN 
MainTable
AS       Parent
      ON Parent.ID = cte.ParentID
     AND Parent.SomeOtherData = cte.SomeOtherData