Re: Patch: add conversion from pg_wchar to multibyte

I wrote:
> Some inspection of pg_wchar.h suggests that the IS_LCPRV1 and IS_LCPRV2
> cases are unused: the file doesn't define any encoding labels that match
> the byte values they accept, nor do the comments suggest that Emacs has
> any such labels either.

Scratch that --- I was misled by the fond illusion that our code
wouldn't use magic hex literals for encoding labels.  Stuff like this:
       /* 0x9d means LCPRV2 */       if (c1 == LC_CNS11643_1 || c1 == LC_CNS11643_2 || c1 == 0x9d)

seems to me to be well below the minimum acceptable quality standards
for Postgres code.

Having said that, grepping the src/backend/utils/mb/conversion_procs/
reveals no sign that 0x9a, 0x9b, or 0x9c are used anywhere with the
meanings that the IS_LCPRV1 and IS_LCPRV2 macros assign to them.
Furthermore, AFAICS the 0x9d case is only used in euc_tw_and_big5/,
with the following byte being one of the LC_CNS11643_[3-7] constants.

Given that these constants are treading on encoding ID namespace that
Emacs upstream might someday decide to assign, I think we'd be well
advised to *not* start installing any code that thinks that 9a-9c
mean something.
        regards, tom lane