Question

Is there any optimizations I can do with this statement to get the count of used rows.

Note: The ProductOrderInfo table has over 40 million rows in it.

SELECT @TotalUsed = COUNT(*)
FROM ProductInfo WITH (NOLOCK)
WHERE ProductInfoId IN 
(
    SELECT ProductInfoId FROM ProductOrderInfo WITH (NOLOCK)
);

Answer 1

Use JOIN for such queries:

SELECT @TotalUsed = COUNT(DISTINCT i.ProductInfoId) 
FROM ProductInfo i WITH (NOLOCK) 
JOIN ProductOrderInfo oi WITH (NOLOCK) 
    ON io.ProductInfoId = i.ProductInfoId

Tables should have Indexes by these columns for fast search.