16.2. 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.) A database cluster is a collection of databases that is managed by a single instance of a running database server. After initialization, a database cluster will contain a database named postgres, which is meant as a default database for use by utilities, users and third party applications. The database server itself does not require the postgres database to exist, but many external utility programs assume it exists. Another database created within each cluster during initialization is called 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 20 for information about creating new databases within a cluster.)

You should initialize database cluster for each Coordinator and Datanode.

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 Postgres-XC. The desired file system location of your database cluster is indicated by the -D option. You also need to define a node name for the cluster element initialized, for example:

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

Note that you must execute this command while logged into the Postgres-XC user account, which is described in the previous section. You should assign separate data directory to each Coordinator and Datanode if you are configuring them in a same server.

Alternatively, you can run initdb via the pg_ctl program like so:

$ pg_ctl -D /usr/local/pgsql/data -o '--nodename foo' initdb

This may be more intuitive if you are using pg_ctl for starting and stopping the server (see Section 16.3), so that pg_ctl would be the sole command you use for managing the database server instance.

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 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 do not trust other local users, we recommend you use one of initdb's -W, --pwprompt or --pwfile options to assign a password to the database superuser. Also, specify -A md5 or -A password so that the default trust authentication mode is not used; or modify the generated pg_hba.conf file after running initdb, but before you start the server for the first time. (Other reasonable approaches include using peer authentication or file system permissions to restrict connections. See Chapter 18 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 21.1. The default sort order used within the particular database cluster is set by initdb, and while you can create new databases using different sort order, the order used in the template databases that initdb creates cannot be changed without dropping and recreating them. There is also a performance impact for using locales other than C or POSIX. Therefore, it is important to make this choice correctly the first time.

initdb also sets the default character set encoding for the database cluster. Normally this should be chosen to match the locale setting. For details see Section 21.3.