Comparing date in subquery prevents SQL Server from using index - sql-server

Question

I'm trying to filter only events that happened within 30 days after the company was created (RegisteredUtc).
Does anyone know how to rewrite this query to make sure that predicate uses the index that exists?
I've tried both with an index that has (eventtype, timeutc) and includes (companyid) and one that has (eventtype) and includes (companyid, timeutc).
This is the query that does what I want. NOTE: between these 3 queries it's only the last predicate in the first where clause that changes.
select
DATEPART(year, q.RegisteredUtc) as [year],
DATEPART(month, q.RegisteredUtc) as [month],
Count(*) as [Count]
from
(select
c.Id,
c.RegisteredUtc,
(select count(*)
from dbo.events sc
where sc.companyId = c.Id
and sc.eventtype = 'CreateInvoice'
and sc.TimeUtc < DATEADD(day, 30, c.RegisteredUtc)) as [Count]
from
dbo.companies c) as q
where
q.Count > 0
group by
DATEPART(year, q.RegisteredUtc), DATEPART(month, q.RegisteredUtc)
But it's very slow because of the and TimeUtc < DATEADD(day, 30, c.RegisteredUtc). Without that it uses the indexes and runs very fast but with that predicate there it does a Eager spool that is very expensive instead of using the index.
There are actually indexes available. Confirmed by simply swapping DATEADD(day, 30, c.RegisteredUtc)with a constant which made the query fast again.
This is also slow.
select
DATEPART(year, q.RegisteredUtc) as [year],
DATEPART(month, q.RegisteredUtc) as [month],
Count(*) as [Count]
from
(select
c.Id,
c.RegisteredUtc,
(select count(*)
from dbo.events sc
where sc.companyId = c.Id
and sc.eventtype = 'CreateInvoice'
and sc.TimeUtc < c.RegisteredUtc) as [Count]
from
dbo.companies c) as q
where
q.Count > 0
group by
DATEPART(year, q.RegisteredUtc), DATEPART(month, q.RegisteredUtc)
This query hits the index and works fast though but of course doesn't produce the correct results.
select
DATEPART(year, q.RegisteredUtc) as [year],
DATEPART(month, q.RegisteredUtc) as [month],
Count(*) as [Count]
from
(select
c.Id,
c.RegisteredUtc,
(select count(*)
from dbo.events sc
where sc.companyId = c.Id
and sc.eventtype = 'CreateInvoice'
and sc.TimeUtc = c.RegisteredUtc) as [Count]
from
dbo.companies c) as q
where
q.Count > 0
group by
DATEPART(year, q.RegisteredUtc), DATEPART(month, q.RegisteredUtc)

Answer 1

As I see it you are not even using the inner count
If it matches the join then the count > 0
select DATEPART(year, q.RegisteredUtc) as [year],
DATEPART(month, q.RegisteredUtc) as [month],
Count(*) as [Count]
from ( select distinct c.Id, c.RegisteredUtc
from dbo.companies c
join dbo.events sc
on sc.companyId = c.Id
and eventtype = 'CreateInvoice'
and TimeUtc < DATEADD(day, 30, c.RegisteredUtc)
) as q
group by DATEPART(year, q.RegisteredUtc), DATEPART(month, q.RegisteredUtc)
where exists may work better. Your query is confusing to me. You get a count just to compare it to > 0 and you are worried about not using an in index?

Answer 2

It might be worthwhile to compute the DATEADD(day, 30, c.RegisteredUtc) in the select and then compare TimeUtc with the computed column, in other words:
select
c.Id,
c.RegisteredUtc,
DATEADD(day, 30, c.RegisteredUtc) as RegisteredUtc30
(
select count(*)
from dbo.events sc
where sc.companyId = c.Id
and sc.eventtype = 'CreateInvoice'
and sc.TimeUtc < RegisteredUtc30
from dbo.companies c