Re: Expression indexes and casts

On Tue, 9 Mar 2004, Tom Lane wrote:

> Stephan Szabo <sszabo@megazone.bigpanda.com> writes:
> > I haven't done any looking around yet (about to head off to work), but it
> > looks like in the case where the system decides to cast a to text in order
> > to get a working equality, the index isn't used, whereas in the case where
> > I explicitly cast it, it can.
>
> I think the problem is that explicit and implicit casts are marked
> differently in the cast parse node, causing equal() to consider the two
> expressions different.
>
> There is currently a hack involving a "don't care" setting for this
> field, but it doesn't help you.  I wonder if it would be better to make
> equal() explicitly ignore the cast-type field.  It seems like that could
> break other things though :-(.
>
> A narrower patch would be to change the cast type field to don't-care in
> the copy of the parse tree that is made for planner user.
>
> [ thinks some more... ]  On the other hand, there are cases where
> explicit and implicit casting are actually semantically different (think
> varchar() and char() length constraints).  Maybe the don't-care business
> is itself a bug, and you're just stuck.

Is it possible to make an index on the implicitly cast version (or what
would that take - I'm not sure how to syntactically note that in any
case)?  I don't really care about the explicit cast case actually.