21.2. Character Set Support

21.2. Character Set Support

The character set support in PostgreSQL allows you to store text in a variety of character sets, 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 character sets can be used transparently throughout the server. (If you use extension functions from other sources, it depends on whether they wrote their code correctly.) The default character set is selected while initializing your PostgreSQL database cluster using initdb. It can be overridden when you create a database using createdb or by using the SQL command CREATE DATABASE. So you can have multiple databases each with a different character set.

21.2.1. Supported Character Sets

Table 21.1, “Server Character Sets” shows the character sets available for use in the server.

Table 21.1. Server Character Sets

Not all APIs support all the listed character sets. For example, the PostgreSQL JDBC driver does not support MULE_INTERNAL, LATIN6, LATIN8, and LATIN10.

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 PostgreSQL will be unable to help you by converting or validating non-ASCII characters.

21.2.2. Setting the Character Set

initdb defines the default character set for a PostgreSQL cluster. For example,

initdb -E EUC_JP

sets the default character set (encoding) to EUC_JP (Extended Unix Code for Japanese). You can use --encoding instead of -E if you prefer to type longer option strings. If no -E or --encoding option is given, initdb attempts to determine the appropriate encoding to use based on the specified or default locale.

You can create a database with a different character set:

createdb -E EUC_KR korean

This will create a database named korean that uses the character set EUC_KR. Another way to accomplish this is to use this SQL command:

CREATE DATABASE korean WITH ENCODING 'EUC_KR';

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

$ psql -l

            List of databases

   Database    |  Owner  |   Encoding

---------------+---------+---------------

 euc_cn        | t-ishii | EUC_CN

 euc_jp        | t-ishii | EUC_JP

 euc_kr        | t-ishii | EUC_KR

 euc_tw        | t-ishii | EUC_TW

 mule_internal | t-ishii | MULE_INTERNAL

 postgres      | t-ishii | EUC_JP

 regression    | t-ishii | SQL_ASCII

 template1     | t-ishii | EUC_JP

 test          | t-ishii | EUC_JP

 utf8          | t-ishii | UTF8

(9 rows)

Important

Although you can specify any encoding you want for a database, it is unwise to choose an encoding that is not what is expected by the locale you have selected. The LC_COLLATE and LC_CTYPE settings imply a particular encoding, and locale-dependent operations (such as sorting) are likely to misinterpret data that is in an incompatible encoding.

Since these locale settings are frozen by initdb, the apparent flexibility to use different encodings in different databases of a cluster is more theoretical than real. It is likely that these mechanisms will be revisited in future versions of PostgreSQL.

One way to use multiple encodings safely is to set the locale to C or POSIX during initdb, thus disabling any real locale awareness.

21.2.3. Automatic Character Set Conversion Between Server and Client

PostgreSQL supports automatic character set conversion between server and client for certain character sets. The conversion information is stored in the pg_conversion system catalog. You can create a new conversion by using the SQL command CREATE CONVERSION. PostgreSQL comes with some predefined conversions. They are listed in Table 21.2, “Client/Server Character Set Conversions”.

Table 21.2. Client/Server Character Set Conversions

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

\encoding SJIS

int PQsetClientEncoding(PGconn *conn, const char *encoding);

where conn is a connection to the server, and encoding is the encoding you want to use. If the function successfully sets the encoding, it returns 0, otherwise -1. The current encoding for this connection can be determined by using:

int PQclientEncoding(const PGconn *conn);

Note that it returns the encoding ID, not a symbolic string such as EUC_JP. To convert an encoding ID to an encoding name, you can use:

char *pg_encoding_to_char(int encoding_id);

SET CLIENT_ENCODING TO 'value';

Also you can use the more 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;

If the conversion of a particular character is not possible [mdash ] suppose you chose EUC_JP for the server and LATIN1 for the client, then some Japanese characters cannot be converted to LATIN1 [mdash ] it is transformed to its hexadecimal byte values in parentheses, e.g., (826C).

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.

21.2.4. Further Reading

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

systems.

http://www.i18ngurus.com/docs/984813247.html

An extensive collection of documents about character sets, encodings, and code pages.

ftp://ftp.ora.com/pub/examples/nutshell/ujip/doc/cjk.inf

Detailed explanations of EUC_JP, EUC_CN, EUC_KR, EUC_TW appear in section 3.2.

http://www.unicode.org/

The web site of the Unicode Consortium

RFC 2044

UTF-8 is defined here.