Cross apply with outer reference forces scan - sql

Question

How do I coerce SQL into seeking against my index in this scenario? I have a cross apply which, if fed static values seeks correctly. If fed input from the outer rows, it fails to generate a plan. What's the difference... should it be able to take the rows from the topmost operator and feed them into the cross apply?
select *
from AccessorGrantPermissableAssociations a
cross apply
(
select z.AccessorId, z.PermissableId, max(z.CreatedDate) CreatedDate
-- notice forceseek (cannot generate query plan when using reference to alias 'a'
-- works fine when provided static values
from AccessorGrant z (forceseek)
where
-- works
z.AccessorId = 1 and z.PermissableId = 1
-- doesn't work
--z.AccessorId = a.AccessorId and z.PermissableId = a.PermissableId
and z.CreatedDate <= cast(switchoffset(#asOfMoment, '-00:00') as datetime2)
group by z.AccessorId, z.PermissableId
) b
I can prove that the index works becuase I can execute the following with a fast seek:
select z.AccessorId, z.PermissableId, max(z.CreatedDate) CreatedDate
from AccessorGrant z (forceseek)
where z.AccessorId = 1 and z.PermissableId = 1
and z.CreatedDate <= cast(switchoffset(#asOfMoment, '-00:00') as datetime2)
group by z.AccessorId, z.PermissableId
For your info, there is an index on AccessorGrant:
(AccessorId, PermissableId, CreatedDate)
Reiterate Question:
Why doesn't the same query work in a cross apply that does if its provided static values? How can I get the most recent date for every pair of AccessibleId and PermissableId with an efficient plan?
update plans (pasteplan didn't work for me)
Here is a plan using z.AccessorId = 1 and z.PermissableId = 1:
Here is a plan using z.AccessorId = a.AccessorId and z.PermissableId = a.PermissableId:

Answer 1

This looks like a slight variation of a classic top-n-per-group problem.
It can be done with CROSS APPLY or with ROW_NUMBER. The best method depends on your data distribution.
If we keep the CROSS APPLY approach, I would rewrite your query like this:
select *
from
AccessorGrantPermissableAssociations AS a
cross apply
(
select TOP(1)
z.AccessorId, z.PermissableId, z.CreatedDate
from
AccessorGrant AS z
where
z.AccessorId = a.AccessorId
and z.PermissableId = a.PermissableId
and z.CreatedDate <= cast(switchoffset(#asOfMoment, '-00:00') as datetime2)
ORDER BY z.CreatedDate DESC
) AS b
;
It produces the same result, but with explicit instruction to the server to get only one row from AccessorGrant for each row from AccessorGrantPermissableAssociations. It looks like optimizer is not smart enough to convert MAX into TOP(1) when it is buried behind sub-query in this case. It can do this transformation in the simple query, but can't in this case.
If it still doesn't do seek, change the index to match the query exactly: (AccessorId, PermissableId, CreatedDate DESC).
Most likely if you write the query in this form you would not need a FORCESEEK hint.

Answer 2

Go back to z.AccessorId = a.AccessorId and z.PermissableId = a.PermissableId in your b subquery, but add WHERE a.AccessorId = 1 and a.PermissibleId = 1 in the main query, and get rid of the FORCESEEK table hint.
select *
from AccessorGrantPermissableAssociations a
cross apply
(
select z.AccessorId, z.PermissableId, max(z.CreatedDate) CreatedDate
from AccessorGrant z
where
z.AccessorId = a.AccessorId and z.PermissableId = a.PermissableId
and z.CreatedDate <= cast(switchoffset(#asOfMoment, '-00:00') as datetime2)
group by z.AccessorId, z.PermissableId
) b
WHERE a.AccessorId = 1 and a.PermissableId = 1
Your CROSS APPLY is evaluated for every row in your outer query, so you want to limit the outer query as much as possible.
But really, are you not after just the max created date? It should more properly be this:
select *
from AccessorGrantPermissableAssociations a
cross apply
(
select max(z.CreatedDate) CreatedDate
from AccessorGrant z
where
z.AccessorId = a.AccessorId and z.PermissableId = a.PermissableId
and z.CreatedDate <= cast(switchoffset(#asOfMoment, '-00:00') as datetime2)
) b
WHERE a.AccessorId = 1 and a.PermissableId = 1