Avoid calling render_sql twice

[lonvia]

If you have a generator of the form

[f(x) for x in foo if f(x)]

then Python has to call function f() twice for every element because it cannot know if f() has side-effects. What you want to do here instead apply a filter to the results.

NB: do you really want to eliminate duplicates here? If not, then it would be faster to return a list instead of a set.

[pnorman]

I think eliminating duplicates is actually a bug, although not one that is possible to hit in practice. A list would be better.

[lonvia]

Changed set to list.