The character set support in PostgreSQL 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 PostgreSQL database cluster using 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 character set must be compatible with the server's LC_CTYPE setting. When LC_CTYPE is C or POSIX, any character set is allowed, but for other settings of LC_CTYPE there is only one character set that will work correctly. Since the LC_CTYPE setting is frozen by initdb, the apparent flexibility to use different encodings in different databases of a cluster is more theoretical than real, except when you select C or POSIX locale (thus disabling any real locale awareness). It is likely that these mechanisms will be revisited in future versions of PostgreSQL.
Table 22-1 shows the character sets available for use in PostgreSQL.
Table 22-1. PostgreSQL Character Sets
|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|
|GBK||Extended National Standard||Simplified Chinese||No||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|
|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|
|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|
|WIN1250||Windows CP1250||Central European||Yes||1|
|WIN1252||Windows CP1252||Western European||Yes||1|
|WIN1258||Windows CP1258||Vietnamese||Yes||1||ABC, TCVN, TCVN5712, VSCII|
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.
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.
If you have selected C or POSIX 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 it 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: On most modern operating systems, PostgreSQL can determine which character set is implied by an LC_CTYPE setting, and it will enforce that only the correct 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 misbehavior of locale-dependent operations such as sorting.
PostgreSQL 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.
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 Table 22-2. You can create a new conversion using the SQL command CREATE CONVERSION.
Table 22-2. 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_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, KOI8, MULE_INTERNAL, UTF8, WIN866, WIN1251|
|KOI8||KOI8, ISO_8859_5, MULE_INTERNAL, UTF8, WIN866, WIN1251|
|LATIN1||LATIN1, MULE_INTERNAL, UTF8|
|LATIN2||LATIN2, MULE_INTERNAL, UTF8, WIN1250|
|LATIN3||LATIN3, MULE_INTERNAL, UTF8|
|LATIN4||LATIN4, MULE_INTERNAL, UTF8|
|MULE_INTERNAL||MULE_INTERNAL, BIG5, EUC_CN, EUC_JP, EUC_KR, EUC_TW, ISO_8859_5, KOI8, LATIN1 to LATIN4, SJIS, WIN866, WIN1250, WIN1251|
|SJIS||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, KOI8, MULE_INTERNAL, UTF8, WIN1251|
|WIN1250||WIN1250, LATIN2, MULE_INTERNAL, UTF8|
|WIN1251||WIN1251, ISO_8859_5, KOI8, MULE_INTERNAL, UTF8, WIN866|
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:
libpq (Section 30.9) 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:
To return to the default 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.
These are good sources to start learning about various kinds of encoding systems.
An extensive collection of documents about character sets, encodings, and code pages.
Detailed explanations of EUC_JP, EUC_CN, EUC_KR, EUC_TW appear in section 3.2.
The web site of the Unicode Consortium
UTF-8 is defined here.