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Creating and Managing Tablespaces

Background

The administrator can use tablespaces to control the layout of disks where a database is installed. This has the following advantages:

  • If the initial disk partition or volume allocated to the database is full and the space cannot be logically increased, you can create and use tablespaces in other partitions until the space is reconfigured.
  • Tablespaces allow the administrator to distribute data based on the schema of database objects, improving system performance.

    • A frequently used index can be placed in a disk having stable performance and high computing speed, such as a solid device.
    • A table that stores archived data and is rarely used or has low performance requirements can be placed in a disk with a slow computing speed.
  • The administrator can use tablespaces to set the maximum available disk space. In this way, when a partition is shared with other data, tablespaces will not occupy excessive space in the partition.
  • Each tablespace corresponds to a file system directory. Assume that *Database node data directory*/pg_location/mount1/path1 is an empty directory for which users have read and write permissions.

    If the tablespace quota management is used, the performance may deteriorate by about 30%. MAXSIZE specifies the maximum quota for each database node. The deviation must be within 500 MB. Determine whether to set a tablespace to its maximum size as required.

Procedure

  • Create a tablespace.

    1. Run the following command to create user jack:

      CREATE USER jack IDENTIFIED BY 'Bigdata@123';

      If the following information is displayed, the user has been created:

      CREATE ROLE
    2. Run the following command to create a tablespace:

      CREATE TABLESPACE fastspace RELATIVE LOCATION 'tablespace/tablespace_1';

      If the following information is displayed, the tablespace has been created:

      CREATE TABLESPACE

      fastspace is the new tablespace, and *Database node data directory*/pg_location/tablespace/tablespace_1 is an empty directory for which users have read and write permissions.

    3. A database system administrator can run the following command to grant the permission of accessing the fastspace tablespace to user jack:

      GRANT CREATE ON TABLESPACE fastspace TO jack;

      If the following information is displayed, the permission has been assigned:

      GRANT
  • Create an object in a tablespace.

    If you have the CREATE permission for the tablespace, you can create database objects in the tablespace, such as tables and indexes.

    Take creating a table as an example:

    • Method 1: Run the following command to create a table in a specified tablespace:

      CREATE TABLE foo(i int) TABLESPACE fastspace;

      If the following information is displayed, the table has been created:

      CREATE TABLE
    • Method 2: Use set default_tablespace to set the default tablespace and then create a table:

      postgres=# SET default_tablespace = 'fastspace';
      SET
      postgres=# CREATE TABLE foo2(i int);
      CREATE TABLE

      In this example, fastspace is the default tablespace, and foo2 is the created table.

  • Use one of the following methods to query a tablespace:

    • Method 1: Check the pg_tablespace system catalog. Run the following command to view all the tablespaces defined by the system and users:

      SELECT spcname FROM pg_tablespace;
    • Method 2: Run the following meta-command of the gsql program to query the tablespaces:

      \db
  • Query the tablespace usage.

    1. Query the current usage of the tablespace.

      SELECT PG_TABLESPACE_SIZE('example');

      Information similar to the following is displayed:

       pg_tablespace_size 
      --------------------
                 2146304
      (1 row)

      2146304 is the size of the tablespace, and its unit is byte.

    2. Calculate the tablespace usage.

      Tablespace usage rate = PG_TABLESPACE_SIZE/Size of the disk where the tablespace resides

  • Modify a tablespace.

    Run the following command to rename tablespace fastspace to fspace:

    postgres=# ALTER TABLESPACE fastspace RENAME TO fspace;
    ALTER TABLESPACE
  • Delete a tablespace and related data.

    • Run the following command to delete user jack:

      postgres=# DROP USER jack CASCADE;
      DROP ROLE
    • Run the following commands to delete tables foo and foo2:

      postgres=# DROP TABLE foo;
      postgres=# DROP TABLE foo2;

      If the following information is displayed, the tables have been deleted:

      DROP TABLE
    • Run the following command to delete tablespace fspace:

      postgres=# DROP TABLESPACE fspace;
      DROP TABLESPACE

      img NOTE: Only the tablespace owner or system administrator can delete a tablespace.