21.2. Character Set Support

21.2.1. Supported Character Sets

Tabela 21-1 shows the character sets available for use in PostgreSQL.

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)

21.2.3. Automatic Character Set Conversion Between Server and Client

PostgreSQL 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. PostgreSQL comes with some predefined conversions, as shown in Tabela 21-2. You can create a new conversion using the SQL command CREATE CONVERSION.

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:

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

  \encoding SJIS
  

• Using libpq functions. \encoding actually calls PQsetClientEncoding() for its purpose.

  int PQsetClientEncoding(PGconn *conexão, const char *codificação);
  

  where conexão is a connection to the server, and codificação 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 *conexão);
  

  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 id_codificação);
  

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

  SET CLIENT_ENCODING TO 'valor';
  

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

  SET NAMES 'valor';
  

  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, then some Japanese characters do not have a representation in LATIN1 — then 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.

21.2.4. Leitura adicional

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