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Re: Patch: add conversion from pg_wchar to multibyte

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Alexander Korotkov <aekorotkov(at)gmail(dot)com>
Cc: Tatsuo Ishii <ishii(at)postgresql(dot)org>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Patch: add conversion from pg_wchar to multibyte
Date: 2012-07-02 20:37:13
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Lists: pgsql-hackers
On Mon, Jul 2, 2012 at 4:33 PM, Alexander Korotkov <aekorotkov(at)gmail(dot)com> wrote:
> On Mon, Jul 2, 2012 at 8:12 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>> On Sun, Jul 1, 2012 at 5:11 AM, Alexander Korotkov <aekorotkov(at)gmail(dot)com>
>> wrote:
>> >> MULE also looks problematic.  The code that you've written isn't
>> >> symmetric with the opposite conversion, unlike what you did in all
>> >> other cases, and I don't understand why.  I'm also somewhat baffled by
>> >> the reverse conversion: it treats a multi-byte sequence beginning with
>> >> a byte for which IS_LCPRV1(x) returns true as invalid if there are
>> >> less than 3 bytes available, but it only reads two; similarly, for
>> >> IS_LCPRV2(x), it demands 4 bytes but converts only 3.
>> >
>> > Should we save existing pg_wchar representation for MULE encoding?
>> > Probably,
>> > we can modify it like in 0.1 version of patch in order to make it more
>> > transparent.
>> Changing the encoding would break pg_upgrade, so -1 from me on that.
> I didn't realize that we store pg_wchar on disk somewhere. I thought it is
> only in-memory representation. Where do we store pg_wchar on disk?

OK, now I'm confused.  I was thinking (incorrectly) that you were
talking about changing the multibyte encoding, which of course is
saved on disk all over the place.  Changing the wchar encoding is a
different kettle of fish, and I have no idea what that would or would
not break.  But I don't see why we'd want to do such a thing.  We just
need to make the MB->WCHAR and WCHAR->MB transformations mirror images
of each other; why is that hard?

Robert Haas
The Enterprise PostgreSQL Company

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