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Creating and Managing Databases

Prerequisites

Only the database system administrator or users granted with database creation permissions can create a database. For details about how to grant database creation permissions to a user, see Managing Users and Their Permissions.

Background

  • MogDB has two default template databases template0 and template1 and a default user database postgres. The default database compatibility type of postgres is O (that is, DBCOMPATIBILITY = A), and the string is treated as NULL under this compatibility type.
  • CREATE DATABASE creates a database by copying a template database (template0 by default). Do not use a client or any other tools to connect to or to perform operations on the template databases.
  • A maximum of 128 databases can be created in MogDB.
  • A database system consists of multiple databases. A client can connect to only one database at a time. Users cannot query data across databases. If an MogDB cluster contains multiple databases, set the -d parameter to specify the database to connect to.

Precautions

Assume that the database encoding is SQL_ASCII. (You can run the show server_encoding; command to query the encoding used for storing data in the current database.) If the database object name contains multi-byte characters (such as Chinese) or if the object name length exceeds the allowed maximum (63 bytes), the database truncates the last byte (not the last character) of the object name. In this case, half characters may appear.

To resolve this problem, you need to:

  • Ensure that the name of the data object does not exceed the maximum length.
  • Use a proper coded character set, such as UTF-8, as the default database storage code set (server_encoding).
  • Exclude multi-byte characters from object names.
  • Ensure that no more than 128 databases are created.
  • If you fail to delete an object by specifying its name after truncation, specify its original name to delete it, or manually delete it from the system catalogs on each node.

Procedure

  1. Create a database.

    1. Run the following command to create a tablespace named tpcds_local:

      postgres=# CREATE TABLESPACE tpcds_local RELATIVE LOCATION 'tablespace/tablespace_1' ;
      CREATE TABLESPACE
    2. Run the following command to create a database named db_tpcc:

      postgres=# CREATE DATABASE db_tpcc WITH TABLESPACE = tpcds_local;
      CREATE DATABASE

    img NOTE:

    • Database names must comply with the general naming convention rules of SQL identifiers. The current role automatically becomes the owner of this new database.
    • If a database system is used to support independent users and projects, store them in different databases.
    • If the projects or users are associated with each other and share resources, store them in one database. However, you can divide them into different schemas. A schema is a logical structure, and the access permission for a schema is controlled by the permission system module.
    • A database name contains a maximum of 63 bytes and the excessive bytes at the end of the name will be truncated by the server. You are advised to specify a database name no longer than 63 bytes when you create a database.
  2. View databases.

    • Run the \l meta-command to view the database list of the database system.

      \l
    • Run the following command to query the database list in the pg_database system catalog:

      SELECT datname FROM pg_database;
  3. Modify the database.

    You can modify database configuration such as the database owner, name, and default settings.

    • Run the following command to set the default search path for the database:

      postgres=# ALTER DATABASE db_tpcc SET search_path TO pa_catalog,public;
      ALTER DATABASE
    • Run the following command to modify the database tablespaces:

      postgres=# ALTER DATABASE db_tpcc SET TABLESPACE tpcds;
      ALTER DATABASE
    • Run the following command to rename the database:

      postgres=# ALTER DATABASE db_tpcc RENAME TO human_tpcds;
      ALTER DATABASE
  4. Delete the database.

    You can run the DROP DATABASE command to delete a database. This command deletes the system directory in the database, as well as the database directory on the disk that stores data. Only the database owner or system administrator can delete a database. A database accessed by users cannot be deleted. You need to connect to another database before deleting this database.

    Run the following command to delete the database:

    postgres=# DROP DATABASE human_tpcds;
    DROP DATABASE