[huntse]

Relational databases really are for doing relational operations. If you're using them for doing bits of arithmetic then you're going to come up against all sorts of limitations. Trivially, if you use this computed value in a where clause the query optimiser will have no choice but to do a full table scan. It can't index by all possible computed values.

Horses for courses. Do the query in whatever programming language, then just multiply the numbers together. I don't try to do left outer joins on my HP12-C calculator either.

[nimbix]

I agree that using a precomputed value in a where clause is a good idea. But it's very likely that computing that value inside a DB engine and fetching a single value is going to be faster than fetching 50 values and calculating the product in your app.

Also, consider the following case: UPDATE huge_table SET someval = (SELECT PRODUCT(x) ...). Splitting this into 50 million queries wouldn't be a very good solution.