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

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

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.