Re: Patch: add conversion from pg_wchar to multibyte

I think it's possible. The first characters are defined like this:

#define IS_LCPRV1(c)    ((unsigned char)(c) == 0x9a || (unsigned char)(c) == 0x9b)
#define IS_LCPRV2(c)    ((unsigned char)(c) == 0x9c || (unsigned char)(c) == 0x9d)

It seems IS_LCPRV1 is not used in any of PostgreSQL supported
encodings at this point, that means there's 0 chance which existing
databases include LCPRV1. So you could safely ignore it.

For IS_LCPRV2, it is only used for Chinese encodings (EUC_TW and BIG5)
in backend/utils/mb/conversion_procs/euc_tw_and_big5/euc_tw_and_big5.c
and it is fixed to 0x9d.  So you can always restore the value to 0x9d.

> Also in this part of code we're shifting first byte by 16 bits:
>
> if (IS_LC1(*from) && len >= 2)
> {
>     *to = *from++ << 16;
>     *to |= *from++;
>     len -= 2;
> }
> else if (IS_LCPRV1(*from) && len >= 3)
> {
>     from++;
>     *to = *from++ << 16;
>     *to |= *from++;
>     len -= 3;
> }
>
> Why don't we shift it by 8 bits?

Because we want the first byte of LC1 case to be placed in the second
byte of wchar. i.e.

0th byte: always 0
1th byte: leading byte (the first byte of the multibyte)
2th byte: always 0
3th byte: the second byte of the multibyte

Note that we always assume that the 1th byte (called "leading byte":
LB in short) represents the id of the character set (from 0x81 to
0xff) in MULE INTERNAL encoding. For the mapping between LB and
charsets, see pg_wchar.h.