A HAWQ system is a single instance of HAWQ. There can be several separate HAWQ systems installed, but usually just one is selected by environment variable settings. See your HAWQ administrator for details.
There can be multiple databases in a HAWQ system. This is different from some database management systems (such as Oracle) where the database instance is the database. Although you can create many databases in a HAWQ system, client programs can connect to and access only one database at a time — you cannot cross-query between databases.
Each new database you create is based on a template. HAWQ provides a default database, template1. Use template1 to connect to HAWQ for the first time. HAWQ uses template1 to create databases unless you specify another template. Do not create any objects in template1 unless you want those objects to be in every database you create.
HAWQ uses two other database templates, template0 and postgres, internally. Do not drop or modify template0 or postgres. You can use template0 to create a completely clean database containing only the standard objects predefined by HAWQ at initialization, especially if you modified template1.
The CREATE DATABASE command creates a new database. For example:
To create a database, you must have privileges to create a database or be a HAWQ superuser. If you do not have the correct privileges, you cannot create a database. The HAWQ administrator must either give you the necessary privileges or to create a database for you.
You can also use the client program createdb to create a database. For example, running the following command in a command line terminal connects to HAWQ using the provided host name and port and creates a database named mydatabase:
$ createdb -h masterhost -p 5432 mydatabase
The host name and port must match the host name and port of the installed HAWQ system.
Some objects, such as roles, are shared by all the databases in a HAWQ system. Other objects, such as tables that you create, are known only in the database in which you create them.
By default, a new database is created by cloning the standard system database template, template1. Any database can be used as a template when creating a new database, thereby providing the capability to ‘clone’ or copy an existing database and all objects and data within that database. For example:
If you are working in the psql client program, you can use the \l meta-command to show the list of databases and templates in your HAWQ system. If using another client program and you are a superuser, you can query the list of databases from the pg_database system catalog table. For example:
=> SELECT datname from pg_database;
The ALTER DATABASE command changes database attributes such as owner, name, or default configuration attributes. For example, the following command alters a database by setting its default schema search path (the search_path configuration parameter):
=> ALTER DATABASE mydatabase SET search_path TO myschema, public, pg_catalog;
To alter a database, you must be the owner of the database or a superuser.
The DROP DATABASE command drops (or deletes) a database. It removes the system catalog entries for the database and deletes the database directory on disk that contains the data. You must be the database owner or a superuser to drop a database, and you cannot drop a database while you or anyone else is connected to it. Connect to template1 (or another database) before dropping a database. For example:
=> \c template1 => DROP DATABASE mydatabase;
You can also use the client program dropdb to drop a database. For example, the following command connects to HAWQ using the provided host name and port and drops the database mydatabase:
$ dropdb -h masterhost -p 5432 mydatabase
Warning: Dropping a database cannot be undone.