What default tablespaces does the oracle database provide?

Oracle's Default Tablespaces: A Comprehensive Guide

This article answers common questions regarding default tablespaces in Oracle databases. We will explore what they are, their characteristics, how to identify them for specific users, and the potential performance implications of their usage.

What default tablespaces does Oracle provide?

Oracle doesn't offer a single, universally defined "default" tablespace. Instead, the specific default tablespaces depend on several factors, primarily the database version and the configuration choices made during database creation. However, some common scenarios and default behaviors include:

• SYSTEM tablespace: This is almost always present. It's a crucial system tablespace containing critical database metadata, dictionary data, and system objects. It's typically created automatically during database creation. While not explicitly a user default, it often acts as the default for system objects and may be the implicit default for certain users if no other default is explicitly set.
• SYSAUX tablespace: Introduced in Oracle 10g, this tablespace stores auxiliary dictionary-managed objects. It's a significant repository for various database components, and its role is to alleviate the load on the SYSTEM tablespace. It also is not typically a user default, but rather a system tablespace.
• USERS tablespace: This is often (but not always) created during database creation, serving as a common default for user-created objects. When a user is created without a specified default tablespace, this USERS tablespace often becomes their default location for storing tables, indexes, and other schema objects. However, this depends on the database creation settings.
• No Explicit Default: In some cases, particularly with custom database setups, a user might not have an explicitly defined default tablespace. In such situations, the database might either fail to create objects for the user, or it might revert to a default (like SYSTEM or SYSAUX), potentially leading to errors or unexpected behavior.

What are the characteristics of the default tablespaces in Oracle?

The characteristics of default tablespaces, particularly SYSTEM and USERS, vary depending on their intended purpose:

• SYSTEM: This tablespace is typically small (relative to other tablespaces), read-heavy, and critically important for database functionality. It often employs a different storage strategy (e.g., smaller block sizes) compared to user tablespaces, and modifications to it should be approached with extreme caution. Direct DDL operations on this tablespace are strongly discouraged.
• SYSAUX: This tablespace is designed to handle a large volume of data and is more flexible in terms of storage management. It's typically larger than the SYSTEM tablespace and has a different organization to handle the diverse objects it stores. It is also critical for database functionality.
• USERS: This tablespace is intended for user-created objects. It usually employs a more standard storage strategy, offering flexibility for data growth. It's the tablespace most frequently modified by users.

How can I identify which tablespace is the default for a specific user in Oracle?

You can identify a user's default tablespace using the following SQL query:

SELECT DEFAULT_TABLESPACE
FROM dba_users
WHERE username = 'your_username';

Replace 'your_username' with the actual username you want to check. If the DEFAULT_TABLESPACE column is NULL, the user doesn't have an explicitly defined default tablespace.

What are the potential performance implications of using the default tablespaces in Oracle?

Using default tablespaces, especially the SYSTEM tablespace, can lead to several performance problems:

• Contention: Multiple users writing to the same default tablespace (often USERS) can cause significant contention for I/O resources, leading to performance degradation for all users.
• Performance Bottlenecks: The SYSTEM tablespace, being crucial for database operations, should not be heavily used for user data. Storing large volumes of user data in SYSTEM or SYSAUX can create bottlenecks and hinder database performance.
• Storage Management: Managing a single, large default tablespace can be complex and difficult, especially as the database grows. Separate tablespaces allow for better storage management, including the ability to use different storage strategies (e.g., different storage types, automatic segment space management) for different types of data.
• Recovery Time: Issues with the SYSTEM or SYSAUX tablespaces can significantly impact database recovery time.

Therefore, it is best practice to create separate tablespaces for different applications and users to avoid these performance implications. While using the default tablespaces might seem convenient initially, it's often a short-sighted approach that can lead to considerable performance challenges in the long run.

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