I have a table like so:
Child
Id
ParentId
ColumnA
ColumnB

What I want is a list of Child Ids such that either ColumnA or ColumnB
is the max (for that column) for a given Parent Id. All columns have
indicies on them.

What I'm using is this:

select Child.Id from child c
join
(select ParentId, max(ColumnA) Max_ColumnA, max(ColumnB) Max_ColumnB
from Child group by ParentId) MaxValues
on MaxValues.ParentId = c.ParentId
where c.ColumnA = MaxValues.Max_ColumnA
or c.ColumnB = MaxValues.Max_ColumnB