Adding an INDEX to a CTE

Question

Should be a pretty straight forward question. Can I add an INDEX to a Common Table Expression (CTE)?

Answer 1

You cannot index a CTE, but the approach is that the CTE can make use of the underlying indexes.

WITH cte AS (
    SELECT myname, SUM(Qty) FROM t GROUP BY myname
)
SELECT *
FROM t a JOIN cte b ON a.myname=b.myname 

In the above query, a JOIN b cannot make use of an index on t.myname because of the GROUP BY.

On the other hand,

WITH cte AS (
    SELECT myname, SUM(Qty) OVER (PARTITION BY myname) AS SumQty, 
                ROW_NUMBER() OVER (PARTITION BY myname ORDER BY myname, Qty) AS n
)
SELECT * FROM t a JOIN cte b ON a.myname=b.myname AND b.n=1 

In the latter query, a JOIN b can make use of an index on t.myname.