Understanding DB2 Tablespaces

DB2 tablespaces are logical storage structures within a DB2 database that provide a layer of abstraction between the database and the physical storage devices. This article will delve into the concept of DB2 tablespaces, their purpose, and provide practical examples of how they are used, focusing on how to create them and relate them to database objects.

Fundamental Concepts / Prerequisites

Before diving into DB2 tablespaces, it's essential to have a basic understanding of the following concepts:

• Database: A structured collection of data.
• Tables: A collection of related data organized in rows and columns within a database.
• Schema: A logical grouping of database objects (tables, views, procedures, etc.).
• Storage Groups: Logical groupings of storage paths where DB2 can store data. Knowledge of DB2's control center (or Data Studio) for creating these is helpful, but the command-line interface (CLI) is demonstrated here.

A working DB2 instance and access to a DB2 command-line interface (CLP) or similar tool are assumed.

Creating and Using Tablespaces in DB2

This section demonstrates creating different types of tablespaces and then creating a table within a newly created tablespace.

-- Connect to the database
CONNECT TO MYDB;

-- Create a regular tablespace with DMS (Database Managed Space) and FILE storage
CREATE REGULAR TABLESPACE TS_REGULAR
  PAGESIZE 8192
  MANAGED BY DATABASE
  USING FILE ('TS_REGULAR_DATA1' 100M, 'TS_REGULAR_DATA2' 100M)
  EXTENTSIZE 32
  PREFETCHSIZE 32
  BUFFERPOOL BP8K;

-- Create a large tablespace with DMS and FILE storage
CREATE LARGE TABLESPACE TS_LARGE
  PAGESIZE 8192
  MANAGED BY DATABASE
  USING FILE ('TS_LARGE_DATA1' 200M, 'TS_LARGE_DATA2' 200M)
  EXTENTSIZE 32
  PREFETCHSIZE 32
  BUFFERPOOL BP8K;

-- Create a temporary tablespace (SMS - System Managed Space)
CREATE TEMPORARY TABLESPACE TS_TEMP
  PAGESIZE 8192
  MANAGED BY SYSTEM
  EXTENTSIZE 32
  PREFETCHSIZE 32
  BUFFERPOOL BP8K;

-- Create a table within the regular tablespace
CREATE TABLE MYTABLE (
  ID INT,
  NAME VARCHAR(255)
) IN TS_REGULAR;

-- Disconnect from the database
CONNECT RESET;

Code Explanation

Let's break down the SQL code step-by-step:

CONNECT TO MYDB;: This command establishes a connection to the database named "MYDB". You'll need to replace "MYDB" with the name of your actual DB2 database.

CREATE REGULAR TABLESPACE TS_REGULAR ...;: This statement creates a regular tablespace named "TS_REGULAR".

• PAGESIZE 8192: Specifies the page size for the tablespace (8KB in this case). Common options are 4096, 8192, 16384, or 32768.
• MANAGED BY DATABASE: Indicates that DB2 will manage the space allocated to this tablespace (DMS).
• USING FILE ('TS_REGULAR_DATA1' 100M, 'TS_REGULAR_DATA2' 100M): Defines the data containers for the tablespace. In this case, two files are created ('TS_REGULAR_DATA1' and 'TS_REGULAR_DATA2'), each with a size of 100MB.
• EXTENTSIZE 32: Specifies the number of pages that will be written to a container before moving to the next container.
• PREFETCHSIZE 32: Specifies the number of pages that will be read into the bufferpool during sequential read operations.
• BUFFERPOOL BP8K: Assigns the tablespace to a specific bufferpool. Bufferpools are regions of memory used to cache table and index data. This should match the PAGESIZE.

CREATE LARGE TABLESPACE TS_LARGE ...;: This statement creates a large tablespace similar to the regular tablespace, but optimized for larger objects. It's defined similarly, using DMS and FILE storage.

CREATE TEMPORARY TABLESPACE TS_TEMP ...;: This statement creates a temporary tablespace. Temporary tablespaces are used to store intermediate results during query processing. The key difference here is MANAGED BY SYSTEM, indicating that the operating system will manage the space (SMS). DMS temporary tablespaces can also be created.

CREATE TABLE MYTABLE ... IN TS_REGULAR;: This statement creates a table named "MYTABLE" and specifies that it should be stored in the "TS_REGULAR" tablespace. The `IN` clause is crucial for placing the table in the desired tablespace.

CONNECT RESET;: Disconnects from the database.

Complexity Analysis

The creation of tablespaces and tables themselves do not have a time complexity in the traditional algorithmic sense. They are administrative operations. However, consider these points:

• Time Complexity: The time taken to create a tablespace depends on the size specified for the data containers and the speed of the underlying storage. Creating larger tablespaces will inherently take longer. Creating a table takes time proportional to the number of columns, indexes and other attributes. The creation itself is generally quite fast. The critical factor is I/O performance of the underlying storage.
• Space Complexity: The space complexity of creating a tablespace is determined by the specified size of the data containers. The system needs to allocate this space on the physical storage devices. For SMS tablespaces, the operating system manages space, but available disk space is still a constraint. For DMS tablespaces, you define the storage size during creation.

Alternative Approaches

An alternative to using DMS with FILE storage is to use DMS with DEVICE storage. Instead of specifying file paths, you specify raw device paths. This can sometimes provide better performance, as it bypasses the file system layer. However, it requires more careful management and typically requires direct DBA involvement to configure the raw devices.

Conclusion

DB2 tablespaces are a fundamental concept for managing data storage within a DB2 database. They provide a layer of abstraction and control, allowing you to optimize performance and manage storage resources effectively. Understanding the different types of tablespaces (regular, large, temporary) and the management methods (DMS, SMS) is essential for designing and administering a DB2 database. This article showed how to create tablespaces and assign a table to a specific tablespace using the command-line interface, the most basic and universal means of accomplishing these tasks.