Databases in PostgreSQL

What Are Databases in PostgreSQL?

A Postgres database - and a computer database in general - is a collection of data stored in computer memory organized specifically for rapid write and read operations.

As already noted Postgres itself is not a database but a database management system (DBMS) or more specifically an relational database management system (RDBMS).

A given Postgres server instance features exactly one database cluster (a collection of databases) which in turn includes at least one database.

Postgres databases data is stored in tables - aka relations ("relation" is a mathematical term for "table").

Postgres database objects are organized by schemas.

Connecting to a Database

Before trying to connect to a Postgres database ensure that a Postgres server is running. The Postgres server process is called postgres. The postgres process manages database objects, performs actions on those objects and accepts connections to databases from client processes.

The postgres process can accept multiple database connections at the same time.

Connecting to a Database with psql from within the Shell

There are several ways of establishing a connection to a Postgres database. One of them is using the psql client that comes with the Postgres server installation.

In addition to executing commands from the shell psql allows - once connected to a database - to execute SQL commands.

Executing simply psql in the shell without any options will attempt to connect with a role name and the database name of the current system user. If no such role or database exists the attempt will fail.

To attempt to establish a connection with explicit role name use -U option.

$ psql -U foo

The above command will attempt to connect as the role with the name foo to the database with the name foo.

To attempt to establish a connection with the explicit database name use -d option.

$ psql -d bar

The above command will attempt to connect to the database name bar with the implicit role name of the system user.

The -U and -d options can be combined.

$ psql -U user -d bar

A shorthand for psql -U user -d bar is psql -U user bar.

To check the name of the currently connected role use the SELECT current_user; command.

To check the name of the currently connected database use the SELECT current_database(); command.

Connecting to a Database with psql from within the psql Prompt

Once connected to a database and being within the psql prompt it is possible to establish the connection to another database (which terminates the connection to the current database) using the \c command.

$ psql bar 

bar=> \c foo;
You are now connected to database "foo" as user "bar".

Connecting to a Database with Non-psql Clients

psql is not the only way to connect to a Postgres database nor is it the primary one.

The primary purpose of any database is to be used as a rapid write and read data storage by any application.

An application (such as a Django, Next.js or Ruby on Rails app) to establish a direct connection to a Postgres database needs to provide:

  • host / socket e.g. 127.0.0.1 or foo.bar.compute.amazonaws.com,

  • port e.g. 5432,

  • role (user) name,

  • role password (if any),

  • database name,

  • other - if necessary (e.g. SSL mode).

In addition it is possible to establish a connection to a remote Postgres database using SSH tunnel.

When the client and the server processes are located on separate hosts the communication is effected over TCP/IP protocol.

Creating a Database

Creating a Database with the createdb Shell Command

To create a database from the shell use the createdb foo command where foo is the database name to be created.

Executing simply createdb will attempt to create a database with the name of the current system user and using the role with the name of the current system user.

Creating a Database with an SQL Command

To create a database with an SQL command use the CREATE DATABASE command.

CREATE DATABASE foo;

Dropping a Database

Dropping a Database with the dropdb Shell Command

To drop a database from the shell use the dropdb foo command.

Dropping a Database with an SQL Command

To drop a database with an SQL command use the DROP DATABASE command.

DROP DATABASE foo;

Listing Databases from within psql

To list all databases from within psql from the current database cluster use the \l internal command.

\l

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