Creating and Managing Databases
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Creating and Managing Databases
A Greenplum Database system is a single instance of Greenplum Database. There can be several separate Greenplum Database systems installed, but usually just one is selected by environment variable settings. See your Greenplum administrator for details.
There can be multiple databases in a Greenplum Database 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 Greenplum system, client programs can connect to and access only one database at a time — you cannot cross-query between databases.
About Template and Default Databases
Greenplum Database provides some template databases and a default database, template1, template0, and postgres.
By default, each new database you create is based on a template database. Greenplum Database uses template1 to create databases unless you specify another template. Creating objects in template1 is not recommended. The objects will be in every database you create using the default template database.
Greenplum Database uses another database template, template0, internally. Do not drop or modify template0. You can use template0 to create a completely clean database containing only the standard objects predefined by Greenplum Database at initialization.
You can use the postgres database to connect to Greenplum Database for the first time. Greenplum Database uses postgres as the default database for administrative connections. For example, postgres is used by startup processes, the Global Deadlock Detector process, and the FTS (Fault Tolerance Server) process for catalog access.
Creating a Database
The CREATE DATABASE command creates a new database. For example:
=> CREATE DATABASE new_dbname;
To create a database, you must have privileges to create a database or be a Greenplum Database superuser. If you do not have the correct privileges, you cannot create a database. Contact your Greenplum Database administrator to either give you the necessary privilege 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 Greenplum Database 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 Greenplum Database system.
Some objects, such as roles, are shared by all the databases in a Greenplum Database system. Other objects, such as tables that you create, are known only in the database in which you create them.
Cloning a Database
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:
=> CREATE DATABASE new_dbname TEMPLATE old_dbname;
Creating a Database with a Different Owner
Another database owner can be assigned when a database is created:
=> CREATE DATABASE new_dbname WITH owner=new_user;
Viewing the List of Databases
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 Greenplum Database 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;
Altering a 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.
Dropping a Database
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 postgres (or another database) before dropping a database. For example:
=> \c postgres => DROP DATABASE mydatabase;
You can also use the client program dropdb to drop a database. For example, the following command connects to Greenplum Database using the provided host name and port and drops the database mydatabase:
$ dropdb -h masterhost -p 5432 mydatabase
The DROP DATABASE command is not transactional.