EUC_JP and SJIS conversion improvement

The character-code conversion from EUC_JP to SJIS is executed by
converting two stages. The first stage is conversion from EUC_JP to MIC.
The next stage is conversion from MIC to SJIS. (Conversion from SJIS to
EUC_JP is also similar.)

It is not so efficient, because it is necessary to allocate the
buffer for MIC, and to execute the calculation for conversion twice.

In the attached patch, it enables the direct conversion of EUC_JP and
SJIS. Additionally, there is an improvement that reduce the call of
pg_mic_mblen.

The effect of the patch that I measured is as follows:

o The Data for test was created by 'pgbench -i'.

o Test SQL:
set client_encoding to 'SJIS';
select * from accounts;

o Test results: Linux(CPU: Pentium III, Compiler option: -O2)
- original: 2.920s
- patched : 2.278s

regards,

---
Atsushi Ogawa

I have tested Atsushi's patches with PostgreSQL 8.0.3 on my Note PC
running Linux 2.4 and got following results (database encoding is
EUC_JP):

1) without patches

$ time psql -c 'set client_encoding to 'SJIS';select * from accounts;' test >/dev/null

real 0m4.926s
user 0m1.680s
sys 0m0.090s

2) with patches

$ time psql -c 'set client_encoding to 'SJIS';select * from accounts;' test >/dev/null

real 0m3.816s
user 0m1.560s
sys 0m0.070s

3) no encoding conversions

$ time psql -c 'set client_encoding to 'EUC_JP';select * from accounts;' test >/dev/null

real 0m3.220s
user 0m1.760s
sys 0m0.070s

I got the 52% overhead decreases to 18% with the patches. This is a
huge improvement! I will commit to current if there's no objection.
--
Tatsuo Ishii