Danny Shemesh <dany74q(at)gmail(dot)com> writes:
> That is of course correct, but what I mean is that, I think that if one
> would explicitly query f(x), and never for x directly, it would've been
> theoretically possible to say that the index is covering for every f(x),
> wouldn't it ?
Theoretically, yeah, but we don't support that: an index-only scan
will only be considered if x itself is available from the index.
There are a couple of reasons for that, but the main one is that
detecting whether an index matches the query would be far more expensive
if it had to consider expression subtrees not just the base Vars.
regards, tom lane