Re: date_trunc() in a specific time zone

I wrote:
> BTW, I'd been hoping that we could avoid rotate-to-local-and-back
> in Vik's desired case, but after further thought I suspect the only
> real optimization that's possible compared to writing it out with
> two AT TIME ZONE constructs is to do the zone name lookup just once.
> As an example, truncating to a day-or-larger boundary could result in
> shifting to a different UTC offset than you started with, due to crossing
> a DST boundary.

Here's a v2 that transposes the code to C so that we can get that
optimization.  I've not tried to time it, but it should actually be
a bit faster than standard date_trunc plus one AT TIME ZONE rotation,
never mind two of them.

			regards, tom lane