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'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
libc-provided locales there is only one character set that will work
correctly.
(On Windows, however, UTF-8 encoding can be used with any locale.)
If you have ICU support configured, ICU-provided locales can be used
with most but not all server-side encodings.
Table 23.1 shows the character sets available for use in PostgreSQL.
Table 23.1. PostgreSQL Character Sets
Not all client 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 (encoding)
for a PostgreSQL cluster. For example,
initdb -E EUC_JP
sets the default character set to
EUC_JP (Extended Unix Code for Japanese). You
can use --encoding instead of
-E if you prefer 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 specify a non-default encoding at database creation time, provided that the encoding is compatible with the selected locale:
createdb -E EUC_KR -T template0 --lc-collate=ko_KR.euckr --lc-ctype=ko_KR.euckr korean
This will create a database named korean that
uses the character set EUC_KR, and locale ko_KR.
Another way to accomplish this is to use this SQL command:
CREATE DATABASE korean WITH ENCODING 'EUC_KR' LC_COLLATE='ko_KR.euckr' LC_CTYPE='ko_KR.euckr' 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
Section 22.3.
The encoding for a database is stored in the system catalog
pg_database. You can see it by using the
psql -l option or the
\l command.
$ psql -l
List of databases
Name | Owner | Encoding | Collation | Ctype | Access Privileges
-----------+----------+-----------+-------------+-------------+-------------------------------------
clocaledb | hlinnaka | SQL_ASCII | C | C |
englishdb | hlinnaka | UTF8 | en_GB.UTF8 | en_GB.UTF8 |
japanese | hlinnaka | UTF8 | ja_JP.UTF8 | ja_JP.UTF8 |
korean | hlinnaka | EUC_KR | ko_KR.euckr | ko_KR.euckr |
postgres | hlinnaka | UTF8 | fi_FI.UTF8 | fi_FI.UTF8 |
template0 | hlinnaka | UTF8 | fi_FI.UTF8 | fi_FI.UTF8 | {=c/hlinnaka,hlinnaka=CTc/hlinnaka}
template1 | hlinnaka | UTF8 | fi_FI.UTF8 | fi_FI.UTF8 | {=c/hlinnaka,hlinnaka=CTc/hlinnaka}
(7 rows)
On most modern operating systems, PostgreSQL
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.
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 23.2. You can create a new
conversion using the SQL command CREATE CONVERSION.
Table 23.2. 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:
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
libpq (Section 34.10) 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.
These are good sources to start learning about various kinds of encoding systems.
Contains detailed explanations of EUC_JP,
EUC_CN, EUC_KR,
EUC_TW.
The web site of the Unicode Consortium.
UTF-8 (8-bit UCS/Unicode Transformation Format) is defined here.