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v3.1

Documentation:v3.1

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Character Set Support

The character set support in MogDB allows you to store text in a variety of character sets (also called encodings), including single-byte character sets such as the ISO 8859 series and multiple-byte character sets such as EUC (Extended Unix Code), UTF-8, and Mule internal code. All supported character sets can be used transparently by clients, but a few are not supported for use within the server (that is, as a server-side encoding). The default character set is selected while initializing your MogDB database cluster using gs_initdb. It can be overridden when you create a database, so you can have multiple databases each with a different character set.

An important restriction, however, is that each database's character set must be compatible with the database's LC_CTYPE (character classification) and LC_COLLATE (string sort order) locale settings. For C or POSIX locale, any character set is allowed, but for other locales there is only one character set that will work correctly. (On Windows, however, UTF-8 encoding can be used with any locale.)

Supported Character Sets

Table1shows the character sets available for use in MogDB.

Table1 MogDB Character Sets

Name Description Language Server? Bytes/Char Aliases
BIG5 Big Five Traditional Chinese No 1-2 WIN950, Windows950
EUC_CN Extended UNIX Code-CN Simplified Chinese Yes 1-3
EUC_JP Extended UNIX Code-JP Japanese Yes 1-3
EUC_JIS_2004 Extended UNIX Code-JP, JIS X 0213 Japanese Yes 1-3
EUC_KR Extended UNIX Code-KR Korean Yes 1-3
EUC_TW Extended UNIX Code-TW Traditional Chinese, Taiwanese Yes 1-3
GB18030 National Standard Chinese Yes 1-4
GBK Extended National Standard Simplified Chinese Yes 1-2 WIN936, Windows936
ISO_8859_5 ISO 8859-5, ECMA 113 Latin/Cyrillic Yes 1
ISO_8859_6 ISO 8859-6, ECMA 114 Latin/Arabic Yes 1
ISO_8859_7 ISO 8859-7, ECMA 118 Latin/Greek Yes 1
ISO_8859_8 ISO 8859-8, ECMA 121 Latin/Hebrew Yes 1
JOHAB JOHAB Korean (Hangul) No 1-3
KOI8R KOI8-R Cyrillic (Russian) Yes 1 KOI8
KOI8U KOI8-U Cyrillic (Ukrainian) Yes 1
LATIN1 ISO 8859-1, ECMA 94 Western European Yes 1 ISO88591
LATIN2 ISO 8859-2, ECMA 94 Central European Yes 1 ISO88592
LATIN3 ISO 8859-3, ECMA 94 South European Yes 1 ISO88593
LATIN4 ISO 8859-4, ECMA 94 North European Yes 1 ISO88594
LATIN5 ISO 8859-9, ECMA 128 Turkish Yes 1 ISO88599
LATIN6 ISO 8859-10, ECMA 144 Nordic Yes 1 ISO885910
LATIN7 ISO 8859-13 Baltic Yes 1 ISO885913
LATIN8 ISO 8859-14 Celtic Yes 1 ISO885914
LATIN9 ISO 8859-15 LATIN1 with Euro and accents Yes 1 ISO885915
LATIN10 ISO 8859-16, ASRO SR 14111 Romanian Yes 1 ISO885916
MULE_INTERNAL Mule internal code Multilingual Emacs Yes 1-4
SJIS Shift JIS Japanese No 1-2 Mskanji, ShiftJIS, WIN932, Windows932
SHIFT_JIS_2004 Shift JIS, JIS X 0213 Japanese No 1-2
SQL_ASCII unspecified (see text) any Yes 1
UHC Unified Hangul Code Korean No 1-2 WIN949, Windows949
UTF8 Unicode, 8-bit all Yes 1-4 Unicode
WIN866 Windows CP866 Cyrillic Yes 1 ALT
WIN874 Windows CP874 Thai Yes 1
WIN1250 Windows CP1250 Central European Yes 1
WIN1251 Windows CP1251 Cyrillic Yes 1 WIN
WIN1252 Windows CP1252 Western European Yes 1
WIN1253 Windows CP1253 Greek Yes 1
WIN1254 Windows CP1254 Turkish Yes 1
WIN1255 Windows CP1255 Hebrew Yes 1
WIN1256 Windows CP1256 Arabic Yes 1
WIN1257 Windows CP1257 Baltic Yes 1
WIN1258 Windows CP1258 Vietnamese Yes 1 ABC, TCVN, TCVN5712, VSCII

Not all client APIs support all the listed character sets. The SQL_ASCII setting behaves considerably differently from the other settings. When the server character set is SQL_ASCII, the server interprets byte values 0-127 according to the ASCII standard, while byte values 128-255 are taken as uninterpreted characters. No encoding conversion will be done when the setting is SQL_ASCII. Thus, this setting is not so much a declaration that a specific encoding is in use, as a declaration of ignorance about the encoding. In most cases, if you are working with any non-ASCII data, it is unwise to use the SQL_ASCII setting because MogDB will be unable to help you by converting or validating non-ASCII characters.

Setting the Character Set

gs_initdb defines the default character set (encoding) for a MogDB cluster. See gs_initdb for details.

You can specify a non-default encoding at database creation time use this SQL command, provided that the encoding is compatible with the selected locale:

CREATE DATABASE chinese WITH ENCODING 'UTF8' LC_COLLATE='en_US.UTF8' LC_CTYPE='en_US.UTF8' TEMPLATE=template0;

Notice that the above commands specify copying the template0 database. When copying any other database, the encoding and locale settings cannot be changed from those of the source database, because that might result in corrupt data. For more information see CREATE DATABASE.

The encoding for a database is stored in the system catalog pg_database. You can see it by using the gsql -l option or the \l command.

$ gsql -l
                              List of databases
   Name    | Owner | Encoding |   Collate   |    Ctype    | Access privileges
-----------+-------+----------+-------------+-------------+-------------------
 chinese   | omm   | UTF8     | en_US.UTF8  | en_US.UTF8  |
 mogdb     | omm   | UTF8     | en_US.UTF-8 | en_US.UTF-8 |
 mogila    | omm   | UTF8     | en_US.UTF-8 | en_US.UTF-8 |
 postgres  | omm   | UTF8     | en_US.UTF-8 | en_US.UTF-8 |
 template0 | omm   | UTF8     | en_US.UTF-8 | en_US.UTF-8 | =c/omm           +
           |       |          |             |             | omm=CTc/omm
 template1 | omm   | UTF8     | en_US.UTF-8 | en_US.UTF-8 | =c/omm           +
           |       |          |             |             | omm=CTc/omm
(6 rows)

Important: On most modern operating systems, MogDB can determine which character set is implied by the LC_CTYPE setting, and it will enforce that only the matching database encoding is used. On older systems it is your responsibility to ensure that you use the encoding expected by the locale you have selected. A mistake in this area is likely to lead to strange behavior of locale-dependent operations such as sorting.

MogDB will allow superusers to create databases with SQL_ASCII encoding even when LC_CTYPE is not C or POSIX. As noted above, SQL_ASCII does not enforce that the data stored in the database has any particular encoding, and so this choice poses risks of locale-dependent misbehavior. Using this combination of settings is deprecated and may someday be forbidden altogether.

Automatic Character Set Conversion Between Server and Client

MogDB supports automatic character set conversion between server and client for certain character set combinations. The conversion information is stored in the pg_conversion system catalog. MogDB comes with some predefined conversions, as shown in Table 2.

Table2 Client/Server Character Set Conversions

Server Character Set Available Client Character Sets
BIG5 not supported as a server encoding
EUC_CN EUC_CN, MULE_INTERNAL, UTF8
EUC_JP EUC_JP, MULE_INTERNAL, SJIS, UTF8
EUC_JIS_2004 EUC_JIS_2004, SHIFT_JIS_2004, UTF8
EUC_KR EUC_KR, MULE_INTERNAL, UTF8
EUC_TW EUC_TW, BIG5, MULE_INTERNAL, UTF8
GB18030 not supported as a server encoding
GBK not supported as a server encoding
ISO_8859_5 ISO_8859_5, KOI8R, MULE_INTERNAL, UTF8, WIN866, WIN1251
ISO_8859_6 ISO_8859_6, UTF8
ISO_8859_7 ISO_8859_7, UTF8
ISO_8859_8 ISO_8859_8, UTF8
JOHAB not supported as a server encoding
KOI8R KOI8R, ISO_8859_5, MULE_INTERNAL, UTF8, WIN866, WIN1251
KOI8U KOI8U, UTF8
LATIN1 LATIN1, MULE_INTERNAL, UTF8
LATIN2 LATIN2, MULE_INTERNAL, UTF8, WIN1250
LATIN3 LATIN3, MULE_INTERNAL, UTF8
LATIN4 LATIN4, MULE_INTERNAL, UTF8
LATIN5 LATIN5, UTF8
LATIN6 LATIN6, UTF8
LATIN7 LATIN7, UTF8
LATIN8 LATIN8, UTF8
LATIN9 LATIN9, UTF8
LATIN10 LATIN10, UTF8
MULE_INTERNAL MULE_INTERNAL, BIG5, EUC_CN, EUC_JP, EUC_KR, EUC_TW, ISO_8859_5, KOI8R, LATIN1 to LATIN4, SJIS, WIN866, WIN1250, WIN1251
SJIS not supported as a server encoding
SHIFT_JIS_2004 not supported as a server encoding
SQL_ASCII any (no conversion will be performed)
UHC not supported as a server encoding
UTF8 all supported encodings
WIN866 WIN866, ISO_8859_5, KOI8R, MULE_INTERNAL, UTF8, WIN1251
WIN874 WIN874, UTF8
WIN1250 WIN1250, LATIN2, MULE_INTERNAL, UTF8
WIN1251 WIN1251, ISO_8859_5, KOI8R, MULE_INTERNAL, UTF8, WIN866
WIN1252 WIN1252, UTF8
WIN1253 WIN1253, UTF8
WIN1254 WIN1254, UTF8
WIN1255 WIN1255, UTF8
WIN1256 WIN1256, UTF8

To enable automatic character set conversion, you have to tell MogDB the character set (encoding) you would like to use in the client. There are several ways to accomplish this:

  • Using the \encoding command in gsql. \encoding allows you to change client encoding on the fly. For example, to change the encoding to SJIS, type:

    \encoding SJIS
  • libpq has functions to control the client encoding.

  • Using SET client_encoding TO. Setting the client encoding can be done with this SQL command:

    SET CLIENT_ENCODING TO 'value';

    Also you can use the standard SQL syntax SET NAMES for this purpose:

    SET NAMES 'value';

    To query the current client encoding:

    SHOW client_encoding;

    To return to the default encoding:

    RESET client_encoding;
  • Using PGCLIENTENCODING. If the environment variable PGCLIENTENCODING is defined in the client's environment, that client encoding is automatically selected when a connection to the server is made. (This can subsequently be overridden using any of the other methods mentioned above.)

  • Using the configuration variable client_encoding. If the client_encoding variable is set, that client encoding is automatically selected when a connection to the server is made. (This can subsequently be overridden using any of the other methods mentioned above.)

If the conversion of a particular character is not possible — suppose you chose EUC_JP for the server and LATIN1 for the client, and some Japanese characters are returned that do not have a representation in LATIN1 — an error is reported.

If the client character set is defined as SQL_ASCII, encoding conversion is disabled, regardless of the server's character set. Just as for the server, use of SQL_ASCII is unwise unless you are working with all-ASCII data.

Further Reading

These are good sources to start learning about various kinds of encoding systems.

  • CJKV Information Processing: Chinese, Japanese, Korean & Vietnamese Computing

    Contains detailed explanations of EUC_JP, EUC_CN, EUC_KR, EUC_TW.

  • http://www.unicode.org/

    The web site of the Unicode Consortium.

  • RFC 3629

    UTF-8 (8-bit UCS/Unicode Transformation Format) is defined here.

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