Transitioning from Oracle to PostgreSQL: Tablespaces

Tablespaces play an important role in database management systems, as they determine where and how database objects like tables and indexes are stored. Both Oracle and PostgreSQL have the concept of tablespaces, but they implement them differently based on the overall architecture of each database.

Oracle’s tablespaces are an integral part of the database that provide various functionalities, including separating data types, managing storage, and optimizing performance. PostgreSQL, on the other hand, takes a more simplified approach, using tablespaces primarily to control where physical files are stored.

This blog aims to provide a comprehensive comparison between Oracle and PostgreSQL tablespaces, covering their architecture, creation, and practical use cases, with the goal of helping DBAs better understand their capabilities and limitations

1. Overview of Tablespaces in Oracle and PostgreSQL

Oracle’s Approach to Tablespaces

In Oracle, tablespaces are logical storage units that consist of one or more data files. These data files store the actual data, such as tables, indexes, and materialized views. Oracle uses several different types of tablespaces to serve specific purposes:

• SYSTEM and SYSAUX: These contain essential administrative data, including system metadata. The SYSTEM tablespace is critical for database operation, while SYSAUX helps offload some of the workload.
• User Tablespaces: These are used for storing application-specific data, giving administrators flexibility in how they organize different datasets.
• TEMP Tablespace: Temporary tablespaces are used to manage temporary data such as sorting results.
• UNDO Tablespace: The UNDO tablespace is used to store undo information, which is essential for transaction rollbacks and maintaining read consistency.

Oracle’s approach allows DBAs to manage storage performance, optimize I/O, and facilitate database maintenance.

PostgreSQL’s Approach to Tablespaces

PostgreSQL tablespaces are much simpler compared to Oracle’s, mainly serving as pointers to directories in the file system where the database stores its data. The default location for storage is the data directory (pg_default), but custom tablespaces can be created to manage storage across different physical disks.

• Default and Custom Tablespaces: PostgreSQL allows users to create tablespaces using the CREATE TABLESPACE command and specify the directory path. These custom tablespaces can be used to store database objects in different storage locations, improving performance by distributing I/O load.

Unlike Oracle, PostgreSQL does not include specialized tablespaces for temporary or undo data; instead, these functions are handled differently within the database, relying on MVCC for read consistency and filesystem directories for storage.

2. Creation and Management of Tablespaces

Oracle Tablespace Management

Creating and managing tablespaces in Oracle involves several advanced features:

• Creating Tablespaces: Oracle tablespaces are created with the CREATE TABLESPACE command. Administrators can specify parameters such as DATAFILE, SIZE, AUTOEXTEND, and extent management to manage how the data is stored.

CREATE TABLESPACE user_data?

DATAFILE '/u01/app/oracle/oradata/orcl/user_data01.dbf' SIZE 100M?

AUTOEXTEND ON NEXT 10M MAXSIZE 1G;Copy to Clipboard        

This command creates a user_data tablespace with a data file that grows as needed.

• Managing Growth: Oracle provides flexibility with AUTOEXTEND and allows adding data files to increase tablespace capacity. The use of bigfile and smallfile tablespaces also helps manage different data requirements, with bigfile tablespaces providing simplified management for large databases.
• Specialized Tablespaces: Temporary (TEMP) and undo (UNDO) tablespaces serve specialized functions, such as managing temporary sort operations or transaction rollback. This segmentation enhances performance and ensures that different operations do not compete for the same storage resources.

PostgreSQL Tablespace Management

In PostgreSQL, tablespaces are created using the CREATE TABLESPACE command and are associated with a specific directory. For example:

CREATE TABLESPACE fast_storage LOCATION '/mnt/ssd/pg_fast_storage';Copy to Clipboard        

This command creates a fast_storage tablespace that points to the /mnt/ssd/pg_fast_storage directory.

• Assigning Tablespaces: PostgreSQL allows tablespaces to be assigned to entire databases, individual tables, or indexes. This flexibility helps DBAs manage storage by distributing data across different physical devices, thus optimizing performance.

Directory-Based Management: Unlike Oracle’s data file system, PostgreSQL tablespaces rely directly on the file system. There is no auto-resizing or extent manag

3. Architectural Differences

Data File Handling

• Oracle: In Oracle, a tablespace consists of one or more data files. These data files can grow automatically or be resized manually. Oracle handles data distribution within a tablespace using extents and segments, which helps with storage optimization and performance.
• PostgreSQL: A PostgreSQL tablespace is represented as a directory in the file system, where all data related to that tablespace is stored. PostgreSQL relies on the file system for data management, without the use of explicit data files as in Oracle.

Purpose and Complexity

• Oracle: Oracle tablespaces serve as logical containers that help separate system, user, temporary, and undo data. This layered architecture provides better control and performance tuning capabilities.
• PostgreSQL: PostgreSQL tablespaces are primarily used for placing data on different storage devices. The simplicity of PostgreSQL’s tablespace architecture is advantageous for DBAs seeking a straightforward storage solution, but it lacks the specialized tablespace options found in Oracle.

4. Practical Usage and Application

Oracle Use Cases

Oracle tablespaces are used for:

• Optimizing I/O: Placing tables and indexes in separate tablespaces reduces contention and enhances performance.
• Scaling Storage: Using bigfile tablespaces helps manage large datasets by reducing the number of physical files that need to be handled.

PostgreSQL Use Cases

PostgreSQL tablespaces are often used to:

• Distribute Data: Spread data across different drives for improved performance and efficient use of storage resources.
• Custom Storage Locations: Assign high-usage tables or indexes to faster storage (e.g., SSD) for better read/write performance.

5. Backup, Recovery, and Maintenance

Backup Strategies

• Oracle: Oracle tablespaces can be backed up individually using RMAN, and point-in-time recovery is available for specific tablespaces. This level of granularity is beneficial for minimizing downtime during maintenance.
• PostgreSQL: PostgreSQL tablespaces are included in backups taken with tools like pg_basebackup. However, PostgreSQL lacks Oracle’s fine-grained recovery options, as tablespaces must be managed as part of the entire database backup.

Maintenance Tasks

• Oracle: Oracle allows taking tablespaces offline for maintenance, changing their status to read-only, or resizing them for future growth—all of which provide flexibility for handling storage efficiently.
• PostgreSQL: Maintenance tasks such as moving a tablespace to a new location require stopping the database and manually relocating the directory. PostgreSQL’s maintenance processes are more dependent on manual intervention compared to Oracle’s automated features.

6. High Availability and Performance Considerations

Performance Optimization

• Oracle: Oracle tablespaces are used to optimize I/O load and manage resources like TEMP and UNDO separately, which is particularly useful in high-concurrency environments.
• PostgreSQL: PostgreSQL tablespaces help distribute I/O across different storage devices, providing performance gains for specific workloads. However, PostgreSQL lacks Oracle’s granularity in separating different types of data operations.

7. Migration Considerations

Migrating from Oracle to PostgreSQL requires careful mapping of tablespace functionalities:

• Mapping Specialized Tablespaces: Oracle’s UNDO and TEMP tablespaces do not have direct equivalents in PostgreSQL, so data management must be adapted accordingly.
• Data Location Management: Custom tablespaces in PostgreSQL can be used to replicate the performance optimizations available in Oracle, but manual intervention is often required for tuning.

Conclusion

Oracle and PostgreSQL offer tablespaces as a way to manage physical data storage, but their implementation differs significantly. Oracle tablespaces are complex, feature-rich, and capable of handling specialized storage needs for system, temporary, and undo data. PostgreSQL takes a more simplified approach, allowing DBAs to control storage locations through file system directories.

Understanding these differences is crucial for DBAs, particularly when planning migrations or optimizing database performance across different systems.

Further Reading

• Transitioning from Oracle to PostgreSQL: Understanding the Concept of Schema
• PostgreSQL is a viable alternative to Oracle – here is why…?
• “But how much does a PostgreSQL license actually cost?” A Frequently Asked Question from Oracle users considering PostgreSQL

Videos

• Webinar: PostgreSQL 101 for Oracle DBAs
• Fireside Chat: Oracle to PostgreSQL Migration?

Packaged Services by Stormatics

• Oracle to PostgreSQL Migration Service