After creating a clustered index on the Months.Monthname (ref my first post) and rearranging the Select as follows, I get about the same performance as Jeff's original select.

SELECT Amount, [Month]

from

(

SELECT

SUM(SomeAmount) AS Amount,

DATENAME(mm,SomeDateTime) AS [Month]

FROM

#MyHead

WHERE

SomeDateTime >= '2010' AND SomeDateTime < '2011'

GROUP BY

DATENAME(mm,SomeDateTime)

) daa

INNER JOIN

months

ON

[monthname] = [Month]

ORDER BY

months.monthcalendarsequence

That was fun 🙂

Thanks Jeff