在多个Postgresql集群中创建角色

Question

I am quite new to PostgreSQL database. We have started to use it just a week back as a part of our project requirements. It took us a while to understand the architecture of the database.

Having read many articles and documents. we have understood the following points:

1. A database cluster in PostgreSQL is a collection of databases
2. With the default installation of the database, there is a default cluster called main which is activated by default
3. We can create and start a new cluster in the database by the following command: pg_createcluster 9.2 cluster1 -d /usr/local/cluster1 --start
4. Unlike users in MySQL, there are roles in postgresql that are cluster wide when created. ie, each cluster can have different roles. Roles are created using the following: createuser --interactive

I have installed postgresql into my ubuntu system. I have also created 5 different clusters apart from the default 'main' cluster. I can list them using pg_lsclusters command. I would want to now create different roles called 'test,test2,dev,dev2,live,live2' into each cluster. When I create a role it doesn't ask me the cluster in which the role is supposed to be created.

Also when performing any operation on the databases, how can I select the particular cluster into which I would want my commands to be executed.

Answer 1

The name "cluster" in PostgreSQL is a bit misleading, as it is not really a cluster at all. A cluster is essentially a file system directory owned by the user running the PostgreSQL system. Under the directory are files and more directories that contain all the data pertaining to the cluster. A single cluster can hold multiple databases, all of whom are controlled by one running instance of PostgreSQL (this instance may also be referred to as the postmaster ). (The cluster can be distributed over different file systems using tablespaces, but that is besides the point here.)

You can indeed have multiple clusters on a single system, but a PostgreSQL instance can only access a single cluster at a time, being specified at the command line when the postmaster is started. The other clusters can be utilized by other instances of PostgreSQL.

Every cluster can contain multiple databases. A cluster can also have multiple role s (user, group), which are shared between all databases on the cluster; roles are never shared between clusters. Nowadays in PostgreSQL you would use the command CREATE ROLE to create users and groups. They have identical permission structures and the basic difference is that users can log in to the database system and groups can not.

Unless your requirements are really specific (ie you have to run a database on a specific server release version and another on another server release version) you should be able to get by with just a single cluster.