Creating a Database Cluster

3.2. Creating a Database Cluster

Before you can do anything, you must initialize a database storage area on disk. We call this a database cluster. (SQL uses the term catalog cluster instead.) A database cluster is a collection of databases is accessible by a single instance of a running database server. After initialization, a database cluster will contain a database named template1. As the name suggests, this will be used as a template for subsequently created databases; it should not be used for actual work. (See Chapter 5 for information about creating databases.)

In file system terms, a database cluster will be a single directory under which all data will be stored. We call this the data directory or data area. It is completely up to you where you choose to store your data. There is no default, although locations such as /usr/local/pgsql/data or /var/lib/pgsql/data are popular. To initialize a database cluster, use the command initdb, which is installed with PostgreSQL. The desired file system location of your database system is indicated by the -D option, for example

$ initdb -D /usr/local/pgsql/data

Note that you must execute this command while logged into the PostgreSQL user account, which is described in the previous section.

Tip: As an alternative to the -D option, you can set the environment variable PGDATA.

initdb will attempt to create the directory you specify if it does not already exist. It is likely that it will not have the permission to do so (if you followed our advice and created an unprivileged account). In that case you should create the directory yourself (as root) and change the owner to be the PostgreSQL user. Here is how this might be done:

root# mkdir /usr/local/pgsql/data
root# chown postgres /usr/local/pgsql/data
root# su postgres
postgres$ initdb -D /usr/local/pgsql/data

initdb will refuse to run if the data directory looks like it it has already been initialized.

Because the data directory contains all the data stored in the database, it is essential that it be secured from unauthorized access. initdb therefore revokes access permissions from everyone but the PostgreSQL user.

However, while the directory contents are secure, the default client authentication setup allows any local user to connect to the database and even become the database superuser. If you don't trust other local users, we recommend you use initdb's -W or --pwprompt option to assign a password to the database superuser. After initdb, modify the pg_hba.conf file to use md5 or password instead of trust authentication before you start the server for the first time. (Other, approaches include using ident authentication or file system permissions to restrict connections. See Chapter 6 for more information.)

initdb also initializes the default locale for the database cluster. Normally, it will just take the locale settings in the environment and apply them to the initialized database. It is possible to specify a different locale for the database; more information about that can be found in Section 7.1. One surprise you might encounter while running initdb is a notice similar to this:

The database cluster will be initialized with locale de_DE.
This locale setting will prevent the use of indexes for pattern matching
operations.  If that is a concern, rerun initdb with the collation order
set to "C".  For more information see the Administrator's Guide.

This is intended to warn you that the currently selected locale will cause indexes to be sorted in an order that prevents them from being used for LIKE and regular-expression searches. If you need good performance in such searches, you should set your current locale to C and re-run initdb, e.g., by running initdb --lc-collate=C. The sort order used within a particular database cluster is set by initdb and cannot be changed later, short of dumping all data, rerunning initdb, and reloading the data. So it's important to make this choice correctly the first time.

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