Re: Cross-type index comparison support in contrib/btree_gin

I wrote:
> We've had multiple requests for $SUBJECT over the years
> ([1][2][3][4][5], and I'm sure my archive search missed some).
> I finally decided to look into what it'd take to make that happen.

I forgot to mention a couple of questions for review:

> ... it turns out that the
> comparePartial() method is only ever applied to compare the original
> query value with an index entry, which means that internally to
> comparePartial() we can apply the proper cross-type comparison
> operator. Our GIN index documentation about comparePartial() doesn't
> quite say that in so many words, but btree_gin was already relying on
> it --- in a very confusing and ill-explained way, if you ask me, but
> it was relying on it.

Should we adjust the documentation of comparePartial() to promise
explicitly that partial_key is the same datum returned by extractQuery?
By my reading, it kind of implies that, but it's not quite black and
white.

> So basically all I had to do was write a bunch of non-error-throwing
> conversion routines and set up some boilerplate infrastructure.

In the 0005 patch, I relied on date2timestamp_opt_overflow and
its siblings where available. But some of the conversions such
as timestamptz-to-timestamp don't have one of those, so I was
forced to copy-and-paste some fairly low-level code. Would it
make sense to refactor the related core routines to expose
xxx2yyy_opt_overflow interfaces, extending what 5bc450629 and
52ad1e659 did? I wouldn't think this worth doing just for
btree_gin's benefit, but if btree_gin needs it maybe some other
extensions could use it too.

regards, tom lane