Configuring Timezone and Localization Settings

Describes the available timezone and localization features of LightDB-A Database.

Parent topic: Installing and Upgrading LightDB-A

Configuring the Timezone

LightDB-A Database selects a timezone to use from a set of internally stored PostgreSQL timezones. The available PostgreSQL timezones are taken from the Internet Assigned Numbers Authority (IANA) Time Zone Database, and LightDB-A Database updates its list of available timezones as necessary when the IANA database changes for PostgreSQL.

LightDB-A Database selects the timezone by matching a PostgreSQL timezone with the value of the TimeZone server configuration parameter, or the host system time zone if TimeZone is not set. For example, when selecting a default timezone from the host system time zone, LightDB-A Database uses an algorithm to select a PostgreSQL timezone based on the host system timezone files. If the system timezone includes leap second information, LightDB-A Database cannot match the system timezone with a PostgreSQL timezone. In this case, LightDB-A Database calculates a “best match” with a PostgreSQL timezone based on information from the host system.

As a best practice, configure LightDB-A Database and the host systems to use a known, supported timezone. This sets the timezone for the LightDB-A Database coordinator and segment instances, and prevents LightDB-A Database from selecting a best match timezone each time the cluster is restarted, using the current system timezone and LightDB-A Database timezone files (which may have been updated from the IANA database since the last restart). Use the gpconfig utility to show and set the LightDB-A Database timezone. For example, these commands show the LightDB-A Database timezone and set the timezone to US/Pacific.

# gpconfig -s TimeZone
# gpconfig -c TimeZone -v 'US/Pacific'

You must restart LightDB-A Database after changing the timezone. The command gpstop -ra restarts LightDB-A Database. The catalog view pg_timezone_names provides LightDB-A Database timezone information.

About Locale Support in LightDB-A Database

LightDB-A Database supports localization with two approaches:

• Using the locale features of the operating system to provide locale-specific collation order, number formatting, and so on.
• Providing a number of different character sets defined in the LightDB-A Database server, including multiple-byte character sets, to support storing text in all kinds of languages, and providing character set translation between client and server.

Locale support refers to an application respecting cultural preferences regarding alphabets, sorting, number formatting, etc. LightDB-A Database uses the standard ISO C and POSIX locale facilities provided by the server operating system. For additional information refer to the documentation of your operating system.

Locale support is automatically initialized when a LightDB-A Database system is initialized. The initialization utility, gpinitsystem, will initialize the LightDB-A array with the locale setting of its execution environment by default, so if your system is already set to use the locale that you want in your LightDB-A Database system then there is nothing else you need to do.

When you are ready to initiate LightDB-A Database and you want to use a different locale (or you are not sure which locale your system is set to), you can instruct gpinitsystem exactly which locale to use by specifying the -n locale option. For example:

$ gpinitsystem -c gp_init_config -n sv_SE

See Initializing a LightDB-A Database System for information about the database initialization process.

The example above sets the locale to Swedish (sv) as spoken in Sweden (SE). Other possibilities might be en_US (U.S. English) and fr_CA (French Canadian). If more than one character set can be useful for a locale then the specifications look like this: cs_CZ.ISO8859-2. What locales are available under what names on your system depends on what was provided by the operating system vendor and what was installed. On most systems, the command locale -a will provide a list of available locales.

Occasionally it is useful to mix rules from several locales, for example use English collation rules but Spanish messages. To support that, a set of locale subcategories exist that control only a certain aspect of the localization rules:

• LC_COLLATE — String sort order
• LC_CTYPE — Character classification (What is a letter? Its upper-case equivalent?)
• LC_MESSAGES — Language of messages
• LC_MONETARY — Formatting of currency amounts
• LC_NUMERIC — Formatting of numbers
• LC_TIME — Formatting of dates and times

If you want the system to behave as if it had no locale support, use the special locale C or POSIX.

The nature of some locale categories is that their value has to be fixed for the lifetime of a LightDB-A Database system. That is, once gpinitsystem has run, you cannot change them anymore. LC_COLLATE and LC_CTYPE are those categories. They affect the sort order of indexes, so they must be kept fixed, or indexes on text columns will become corrupt. LightDB-A Database enforces this by recording the values of LC_COLLATE and LC_CTYPE that are seen by gpinitsystem. The server automatically adopts those two values based on the locale that was chosen at initialization time.

The other locale categories can be changed as desired whenever the server is running by setting the server configuration parameters that have the same name as the locale categories (see the LightDB-A Database Reference Guide for more information on setting server configuration parameters). The defaults that are chosen by gpinitsystem are written into the coordinator and segment postgresql.conf configuration files to serve as defaults when the LightDB-A Database system is started. If you delete these assignments from the coordinator and each segment postgresql.conf files then the server will inherit the settings from its execution environment.

Note that the locale behavior of the server is determined by the environment variables seen by the server, not by the environment of any client. Therefore, be careful to configure the correct locale settings on each LightDB-A Database host (coordinator and segments) before starting the system. A consequence of this is that if client and server are set up in different locales, messages may appear in different languages depending on where they originated.

Inheriting the locale from the execution environment means the following on most operating systems: For a given locale category, say the collation, the following environment variables are consulted in this order until one is found to be set: LC_ALL, LC_COLLATE (the variable corresponding to the respective category), LANG. If none of these environment variables are set then the locale defaults to C.

Some message localization libraries also look at the environment variable LANGUAGE which overrides all other locale settings for the purpose of setting the language of messages. If in doubt, please refer to the documentation for your operating system, in particular the documentation about gettext, for more information.

Native language support (NLS), which enables messages to be translated to the user’s preferred language, is not enabled in LightDB-A Database for languages other than English. This is independent of the other locale support.

Locale Behavior

The locale settings influence the following SQL features:

• Sort order in queries using ORDER BY on textual data
• The ability to use indexes with LIKE clauses
• The upper, lower, and initcap functions
• The to_char family of functions

The drawback of using locales other than C or POSIX in LightDB-A Database is its performance impact. It slows character handling and prevents ordinary indexes from being used by LIKE. For this reason use locales only if you actually need them.

Troubleshooting Locales

If locale support does not work as expected, check that the locale support in your operating system is correctly configured. To check what locales are installed on your system, you may use the command locale -a if your operating system provides it.

Check that LightDB-A Database is actually using the locale that you think it is. LC_COLLATE and LC_CTYPE settings are determined at initialization time and cannot be changed without redoing gpinitsystem. Other locale settings including LC_MESSAGES and LC_MONETARY are initially determined by the operating system environment of the coordinator and/or segment host, but can be changed after initialization by editing the postgresql.conf file of each LightDB-A coordinator and segment instance. You can check the active locale settings of the coordinator host using the SHOW command. Note that every host in your LightDB-A Database array should be using identical locale settings.

Character Set Support

The character set support in LightDB-A Database 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 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 LightDB-A Database array using gpinitsystem. It can be overridden when you create a database, so you can have multiple databases each with a different character set.

Setting the Character Set

gpinitsystem defines the default character set for a LightDB-A Database system by reading the setting of the ENCODING parameter in the gp_init_config file at initialization time. The default character set is UNICODE or UTF8.

You can create a database with a different character set besides what is used as the system-wide default. For example:

=> CREATE DATABASE korean WITH ENCODING 'EUC_KR';

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 gpinitsystem, the apparent flexibility to use different encodings in different databases is more theoretical than real.

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

Character Set Conversion Between Server and Client

LightDB-A Database supports automatic character set conversion between server and client for certain character set combinations. The conversion information is stored in the coordinator pg_conversion system catalog table. LightDB-A Database comes with some predefined conversions or you can create a new conversion using the SQL command CREATE CONVERSION.

To enable automatic character set conversion, you have to tell LightDB-A Database 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, which allows you to change client encoding on the fly.

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

  => SET CLIENT_ENCODING TO '<value>';
  

  To query the current client encoding:

  => SHOW client_encoding;
  

  To return to the default encoding:

  => RESET client_encoding;
  

• Using the PGCLIENTENCODING environment variable. When 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.)

• Setting the configuration parameter client_encoding. If client_encoding is set in the coordinator postgresql.conf file, that client encoding is automatically selected when a connection to LightDB-A Database 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 deactivated, regardless of the server’s character set. The use of SQL_ASCII is unwise unless you are working with all-ASCII data. SQL_ASCII is not supported as a server encoding.

1 Not all APIs support all the listed character sets. For example, the JDBC driver does not support MULE_INTERNAL, LATIN6, LATIN8, and LATIN10. 2 The SQL_ASCII setting behaves considerably differently from the other settings. Byte values 0-127 are interpreted according to the ASCII standard, while byte values 128-255 are taken as uninterpreted characters. If you are working with any non-ASCII data, it is unwise to use the SQL_ASCII setting as a client encoding. SQL_ASCII is not supported as a server encoding.