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Update multi-byte support README
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‎doc/README.mb

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postgresql 6.5.1 multi-byte (MB) support READMEJuly 11 1999
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PostgreSQL 7.0 multi-byte (MB) support READMEMar 22 2000
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Tatsuo Ishii
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t-ishii@sra.co.jp
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ishii@postgresql.org
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http://www.sra.co.jp/people/t-ishii/PostgreSQL/
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0. Introduction
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The MB support is intended for allowing PostgreSQL to handle
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multi-byte character sets such as EUC(Extended Unix Code), Unicode and
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Mule internal code. With the MB enabled you can use multi-byte
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character sets in regexp ,LIKE and some functions. The default
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character sets in regexp ,LIKE and someotherfunctions. The default
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encoding system chosen is determined while initializing your
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PostgreSQL installation using initdb(1). Note that this can be
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overridden when you create a database using createdb(1) orcreate
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database SQL command. So you could have multiple databases with
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different encodingsystems.
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overridden when you create a database using createdb(1) orby using a
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createdatabase SQL command. So you could have multiple databases with
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eachdifferent encodingsystem.
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MB also fixes some problems concerning with 8-bit single byte
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character sets including ISO8859. (I would not say all of problems
@@ -24,11 +24,11 @@ me know if you find any problem while using 8-bit characters)
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1. How to use
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run configure withthe mb option:
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run configure witha multibyte option:
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% configure --with-mb=encoding_system
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%./configure --enable-multibyte[=encoding_system]
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where encoding_system is one of:
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wheretheencoding_system is one of:
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SQL_ASCIIASCII
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EUC_JPJapanese EUC
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Example:
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% configure --with-mb=EUC_JP
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%./configure --enable-multibyte=EUC_JP
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IfMB is disabled, nothingischanged except better supporting for
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8-bit single byte character sets.
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Ifthe encoding systemisomitted (./configure --enable-multibyte),
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SQL_ASCII is assumed.
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2. How to set encoding
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2. How to settheencoding
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initdb command defines the default encoding for a PostgreSQL
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installation. For example:
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% initdb -e EUC_JP
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% initdb -E EUC_JP
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sets the default encoding to EUC_JP(Extended Unix Code for Japanese).
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Note that you can use "-pgencoding" instead of "-e" if you like longer
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option string:-) If no -e or -pgencoding option is given, the encoding
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Note that you can use "--encoding" instead of "-E" if you like longer
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option string:-) If no -E or --encoding option is given, the encoding
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specified at the compile time is used.
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You can create a database with a different encoding.
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CREATE DATABASE korean WITH ENCODING = 'EUC_KR';
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The encoding for a database is represented as "encoding" column in the
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pg_database system catalog.
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pg_database system catalog. You can see that by using -l or \l of psql
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command.
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datname |datdba|encoding|datpath
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-------------+------+--------+-------------
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template1 | 1739| 1|template1
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postgres | 1739| 0|postgres
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euc_jp | 1739| 1|euc_jp
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euc_kr | 1739| 3|euc_kr
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euc_cn | 1739| 2|euc_cn
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unicode | 1739| 5|unicode
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mule_internal| 1739| 6|mule_internal
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$ psql -l
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List of databases
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Database | Owner | Encoding
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---------------+---------+---------------
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euc_cn | t-ishii | EUC_CN
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euc_jp | t-ishii | EUC_JP
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euc_kr | t-ishii | EUC_KR
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euc_tw | t-ishii | EUC_TW
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mule_internal | t-ishii | MULE_INTERNAL
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regression | t-ishii | SQL_ASCII
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template1 | t-ishii | EUC_JP
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test | t-ishii | EUC_JP
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unicode | t-ishii | UNICODE
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(9 rows)
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A number in the encoding column is "encoding id" and can be translated
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to the encoding name using pg_encoding command.
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3. Automatic encoding translation between backend and frontend
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$ pg_encoding 1
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EUC_JP
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PostgreSQL supports an automatic encoding translation between backend
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and frontend for some encodings.
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If an argument to pg_encoding is not a number, then it is regarded as
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an encoding name and pg_encoding will return the encoding id.
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encoding of backendavailable encoding of frontend
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--------------------------------------------------------------------
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EUC_JPEUC_JP, SJIS
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EUC_TWEUC_TW, BIG5
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LATIN2LATIN2, WIN1250
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LATIN5LATIN5, WIN, ALT
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MULE_INTERNALEUC_JP, SJIS, EUC_KR, EUC_CN,
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EUC_TW, BIG5, LATIN1 to LATIN5,
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WIN, ALT, WIN1250
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$ pg_encoding EUC_JP
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1
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To enable the automatic encoding translation, you have to tell
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PostgreSQL the encoding you would like to use in frontend. There are
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several ways to accomplish this.
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3. PGCLIENTENCODING
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o using \encoding command in psql
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If an environment variable PGCLIENTENCODING is defined on the
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frontend, automatic encoding translation is done by the backend. For
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example, if the backend has been compiled with MB=EUC_JP and
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PGCLIENTENCODING=SJIS(Shift JIS: yet another Japanese encoding
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system), then any SJIS strings coming from the frontend would be
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translated to EUC_JP before going into the parser. Outputs from the
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backend would be translated to SJIS of course.
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\encoding allows you to change frontend encoding on the fly. For
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example, to change the encoding to SJIS, type:
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Supported encodings for PGCLIENTENCODING are:
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\encoding SJIS
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SQL_ASCIIASCII
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EUC_JPJapanese EUC
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SJISYet another Japanese encoding
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EUC_CNChinese EUC
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EUC_KRKorean EUC
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EUC_TWTaiwan EUC
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BIG5Traditional Chinese
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MULE_INTERNALMule internal
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LATIN1ISO 8859-1 English and some European languages
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LATIN2ISO 8859-2 English and some European languages
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LATIN3ISO 8859-3 English and some European languages
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LATIN4ISO 8859-4 English and some European languages
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LATIN5ISO 8859-5 English and some European languages
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KOI8KOI8-R
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WINWindows CP1251
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ALTWindows CP866
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WIN1250Windows CP1250 (Czech)
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o using libpq functions
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Note that UNICODE is not supported(yet). Also note that the
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translation is not always possible. Suppose you choose EUC_JP for the
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backend, LATIN1 for the frontend, then some Japanese characters cannot
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be translated into latin. In this case, a letter cannot be represented
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in the Latin character set, would be transformed as:
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\encoding actually calls PQsetClientEncoding() for its purpose.
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(HEXA DECIMAL)
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int PQsetClientEncoding(PGconn *conn, const char *encoding)
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conn is a connection to the backend, and encoding is an encoding you
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want to use. If it successfully sets the encoding, it returns 0,
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otherwise -1. The current encoding for this connection can be shown by
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using:
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int PQclientEncoding(const PGconn *conn)
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Note that it returns the "encoding id," not the encoding symbol string
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such as "EUC_JP." To convert an encoding id to an encoding symbol, you
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can use:
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char *pg_encoding_to_char(int encoding_id)
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o using PGCLIENTENCODING
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If an environment variable PGCLIENTENCODING is defined in the
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frontend, an automatic encoding translation is done by the backend.
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3. SET CLIENT_ENCODING TO command
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o using SET CLIENT_ENCODING TO command
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Actually setting the frontend side encoding information is done by a
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new command:
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Setting the frontend side encoding can be done a SQL command:
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SET CLIENT_ENCODING TO 'encoding';
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where encoding is one of the encodings those can be set to
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PGCLIENTENCODING. Also you can use SQL92 syntax "SET NAMES" for this
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purpose:
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Also you can use SQL92 syntax "SET NAMES" for this purpose:
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SET NAMES 'encoding';
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RESET CLIENT_ENCODING;
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This would reset the frontend encoding to same as the backend
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encoding, thus no encoding translation would be performed.
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4. About Unicode
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4. References
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An automatic encoding translation between Unicode and any other
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encodings is not supported (yet).
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5. What happens if the translation is not possible?
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Suppose you choose EUC_JP for the backend, LATIN1 for the frontend,
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then some Japanese characters could not be translated into LATIN1. In
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this case, a letter cannot be represented in the LATIN1 character set,
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would be transformed as:
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(HEXA DECIMAL)
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6. References
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These are good sources to start learning various kind of encoding
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systems.
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5. History
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Mar 22, 2000
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* Add new libpq functions PQsetClientEncoding, PQclientEncoding
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* ./configure --with-mb=EUC_JP
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now deprecated. use
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./configure --enable-multibyte=EUC_JP
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instead
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* Add SQL_ASCII regression test case
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* Add SJIS User Defined Character (UDC) support
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* All of above will appear in 7.0
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July 11, 1999
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* Add support for WIN1250 (Windows Czech) as a client encoding
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(contributed by Pavel Behal)

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