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.
You can use tablespaces to control the disk space occupied by data in a database. If the usage of a disk where a tablespace resides reaches 90%, the database switches to the read-only mode. It switches back to read/write mode when the disk usage becomes less than 90%.
You are advised to use the background monitoring program or Database Manager to monitor the disk space usage when using the database to prevent the database from switching to the read-only mode.
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.

GaussDB provides two tablespaces: pg_default and pg_global.

Default tablespace pg_default: stores non-shared system tables, user tables, user table indexes, temporary tables, temporary table indexes, and internal temporary tables. The corresponding storage directory is the base directory in the instance data directory.
Shared tablespace pg_global: stores shared system tables. The corresponding storage directory is the base directory in the global data directory.

Precautions

You are not advised to use user-defined tablespaces in scenarios such as Huawei Cloud. This is because user-defined tablespaces are usually used with storage media other than the main storage (storage device where the default tablespace is located, such as a disk) to isolate I/O resources that can be used by different services. Storage devices use standard configurations and do not have other available storage media in scenarios such as Huawei Cloud. If the user-defined tablespace is not properly used, the system cannot run stably for a long time and the overall performance is affected. Therefore, you are advised to use the default tablespace.

Procedure

Create a tablespace.

Run the following command to create user jack:

        
             openGauss=# CREATE USER jack IDENTIFIED BY 'xxxxxxxxx';

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

        
             CREATE ROLE

Run the following command to create a tablespace:

        
             openGauss=# 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.

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

        
             openGauss=# 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 on 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:

       
            openGauss=# 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:

       
            openGauss=# SET default_tablespace = 'fastspace';
SET
openGauss=# 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:
  1
  
  openGauss=# SELECT spcname FROM pg_tablespace;
- Method 2: Run the following meta-command of the gsql program to query the tablespaces:
```
openGauss=# \db
```

Query the tablespace usage.

Query the current usage of the tablespace.

       
            openGauss=# SELECT PG_TABLESPACE_SIZE('example');

The following information is displayed:

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

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

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:

      
           openGauss=# ALTER TABLESPACE fastspace RENAME TO fspace;
ALTER TABLESPACE

Delete a tablespace and related data.

Run the following command to delete user jack:

       
            openGauss=# DROP USER jack CASCADE;
DROP ROLE

Run the following commands to delete tables foo and foo2:

       
            openGauss=# DROP TABLE foo;
openGauss=# DROP TABLE foo2;

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

       
            DROP TABLE

Run the following command to delete tablespace fspace:
1 2

openGauss=# DROP TABLESPACE fspace; DROP TABLESPACE
Only the tablespace owner or system administrator can delete a tablespace.