Object Mobility in PostgreSQL: How to Move Objects Between Tablespaces


PostgreSQL is a powerful, open-source relational database management system (RDBMS) that allows users to store and manage large volumes of structured data. One of the key features of PostgreSQL is object mobility, which refers to the ability to move objects such as tables, indexes, and tablespaces between different physical locations or storage devices. This capability provides significant benefits in terms of disk space management, performance optimization, and backup and recovery strategies.

Explanation of Object Mobility in PostgreSQL

In PostgreSQL, an object is any entity that can be created or manipulated within a database. Objects include tables, indexes, views, sequences, procedures (or functions), triggers, schemas and tablespaces. Object mobility refers to the ability to move these objects between tablespaces without affecting their functionality or relationships with other objects in the database.

For example, moving a table from one tablespace to another can free up disk space on one device while improving performance on another device with faster I/O capabilities. Similarly, moving an index from one tablespace to another can help optimize query execution times by storing frequently accessed data on faster storage devices.

Importance of Moving Objects Between Tablespaces

The importance of object mobility in PostgreSQL lies mainly in its ability to help users manage their database environments more efficiently and effectively. By moving objects between tablespaces based on specific criteria such as usage patterns or storage requirements users can: – Optimize disk space utilization: Moving rarely used data objects onto slower storage media can free up valuable fast storage for high-priority business-critical data.

– Improve performance: By placing frequently accessed or high-priority tables and indexes on faster disks or solid-state drives (SSDs), performance gains can be realized. – Enable backup and recovery strategies: In case of hardware failure or disaster recovery scenarios redistributing critical components across multiple disks means that backups are easily made and restored.

Brief Overview of the Article

This article provides a comprehensive guide on how to move objects between tablespaces in PostgreSQL. The article starts by defining tablespaces in PostgreSQL and highlighting their advantages and disadvantages. It then examines the reasons for moving objects between tablespaces, including disk space management, performance optimization, and backup and recovery strategies.

The article goes on to outline the necessary steps involved in preparing to move objects between tablespaces, including checking object dependencies, creating a new tablespace or identifying an existing one, granting necessary privileges. The article provides a detailed step-by-step guide to moving objects between tablespaces using practical examples.

Understanding Tablespaces in PostgreSQL

Tablespaces in PostgreSQL are logical containers that allow you to store database objects (tables, indexes, etc.) on different physical devices or file systems. A tablespace is a directory on the file system that contains data files and related metadata.

These data files can be distributed across different devices to optimize performance and storage capacity. In PostgreSQL, tablespaces are created at the cluster level and can be used by multiple databases within that cluster.

Definition of Tablespaces

In PostgreSQL, tablespaces are locations on disk where the database objects can be stored. By default, all objects are stored in the “pg_global” tablespace which is created during installation of PostgreSQL. However, it is recommended to create additional tablespaces and move objects based on their size or usage patterns to better manage disk usage.

Types of Tablespaces

There are two types of tablespaces: 1) Regular tablespace: This is a normal type of tablespace that stores all database objects.

2) Shared tablespace: This type of tablespace stores large non-database directories like log files or backup archives. In addition, there is one special type called “pg_global” which holds global database information for system catalogs.

Advantages and Disadvantages

The advantages of using tablespaces include: – Improved performance – By storing frequently accessed tables on separate disks or file systems from other tables, you can reduce contention for I/O resources. – Flexibility – You don’t have to manage all your tables in one place; you can distribute them across multiple disks or file systems.

– Easier backup and restore – You can backup just a single tablespace instead of backing up entire database clusters. The disadvantages include:

– Additional management overhead – You have to manually move tables between tablespaces when necessary. – Increased complexity – Using tablespaces can make the database system more complex and difficult to manage.

– Higher risk of losing data – If one of the file systems containing a tablespace fails, all objects in that tablespace could be lost. Overall, using tablespaces can provide significant benefits in terms of performance and storage management, but they require careful planning and management to avoid potential risks.

Reasons for Moving Objects Between Tablespaces

Disk space management

One of the most common reasons for moving objects between tablespaces is disk space management. As a PostgreSQL database grows, it can take up a significant amount of disk space. If a tablespace becomes full or runs out of available disk space, it can cause issues with the performance and stability of the database.

By moving objects to a tablespace with more available disk space, you can avoid these issues and ensure that your database runs smoothly. It’s also important to note that different tablespaces can have different I/O characteristics.

For example, solid-state drives (SSDs) may provide faster read and write speeds than traditional hard drives. By moving certain objects to an SSD-backed tablespace, you can improve the overall performance of those objects.

Performance optimization

In addition to disk space management, performance optimization is another key reason for moving objects between tablespaces in PostgreSQL. By placing frequently accessed or high-traffic tables in their own tablespace on a separate disk array or SSD volume, you can optimize their I/O performance without affecting the rest of the database. Similarly, if certain tables or indexes are no longer being used as frequently as they once were, you may choose to move them to a less expensive or lower-performance storage tier (such as an archive tier) in order to free up resources for other parts of the database.

Backup and recovery strategies

Backup and recovery strategies are another important consideration when it comes to object mobility in PostgreSQL. In some cases, you may want to create multiple copies of your data for redundancy purposes.

By placing these copies in different tablespaces on separate disks or volumes – ideally in different physical locations – you can reduce the risk of data loss due to hardware failures or disasters such as fires or floods. Additionally, if you need to restore a specific object or set of objects from a backup, having them stored separately in their own tablespace can simplify the recovery process.

You can restore just the necessary objects without affecting the rest of the database. Overall, understanding the reasons for moving objects between tablespaces is an important part of managing a PostgreSQL database effectively and efficiently.

Preparing to Move Objects Between Tablespaces

Checking Object Dependencies

Before attempting to move an object from one tablespace to another, it is crucial to check its dependencies. These dependencies include all the objects that depend on the object being moved and the objects on which it depends.

The most straightforward way of checking an object’s dependencies is by using the “pg_depend” system catalog table. The pg_depend table stores all the dependencies for a given object in PostgreSQL, including those that are not visible in other system catalogs.

One way of identifying an object’s dependencies is through SQL queries. For instance, you can use the “SELECT * FROM pg_depend WHERE objid = ‘your_object_id’;” command to query all objects that depend on your_object_id and those on which it depends.

If any dependent objects exist, they must be handled before moving the original object. If a dependent object does not exist in one of your tablespaces, you should move it first before moving the other objects.

Creating a New Tablespace or Identifying an Existing One

After checking for object dependencies, you need to identify or create a target tablespace where your objects will reside after being moved from their current location. Creating a new tablespace requires superuser privileges or membership in the db_owner role. To create a new tablespace, use the CREATE TABLESPACE command followed by specifying its physical location and any other necessary parameters such as its size and owner:

CREATE TABLESPACE new_tablespace LOCATION ‘/path/to/new/table/space’; Alternatively, you can identify an existing tablespace by querying PostgreSQL’s system catalog tables using SQL commands such as “SELECT spcname FROM pg_tablespace WHERE spcname = ‘existing_tablespace_name’;”

Granting Necessary Privileges

After identifying or creating your target tablespace, you need to grant the necessary privileges to move your objects to their new location. At a minimum, these privileges include the ability to create tables in the target tablespace. If you are moving objects that depend on other objects, you may also need to grant additional privileges such as ownership of those dependent objects.

To grant privileges, you can use the GRANT command followed by specifying the necessary permissions and target object. For instance, if you want to grant table creation privileges for a user in a specific tablespace, use:

GRANT CREATE ON TABLESPACE new_tablespace TO user; Similarly, granting ownership of dependent objects requires superuser or db_owner privileges and can be done using the ALTER OWNED BY command:

ALTER OWNED BY owner_of_dependent_object TO new_owner; By checking object dependencies, identifying or creating a target tablespace and granting necessary privileges before moving your objects between tablespaces, you ensure that your PostgreSQL databases remain well-organized and perform optimally while preserving data integrity.

Moving Objects Between Tablespaces: Step-by-Step Guide

Preparing the Source Database for Object Mobility

Before moving any objects between tablespaces, it is crucial to prepare the source database properly. This includes exporting data from the existing objects, dropping original database objects, and removing any remaining files associated with these objects.

To extract data from the source database, use the `pg_dump` command-line tool. This tool extracts a PostgreSQL SQL script that represents all data in the tables of your database.

Additionally, if you want to exclude certain schema or tables from your backup file, you can specify those details using parameters such as `–exclude-schema` or `–exclude-table-data`. It is essential to have a well-prepared backup file before proceeding further.

After creating a backup file, it’s time to drop original database objects. Before doing so, ensure that there are no active connections running on these objects using the `pg_stat_activity` view.

Modify connection strings on applications accessing those objects and shut them down before proceeding further to avoid errors while dropping them. Remove any remaining files associated with dropped tablespace locations.

In some cases where there were custom scripts or large size files stored in tablespace directories like pg_tblspc/ or pg_xlog/ may remain on disk after deleting them from PgAdmin console. Deleting those files frees up disk space and avoids future confusion.

Importing Data into a New Database Location

Now that we have prepared our existing database for object mobility let’s import this data into our new desired location. Firstly create a new tablespace location where you want to store your data using CREATE TABLESPACE statement in SQL syntax and assign ownership based on necessity (using ALTER TABLESPACE). Next step is creating a new PostgreSQL cluster by initializing an empty data directory structure with initdb utility, which sets up the configuration files and data storage area for a new installation.

Import previously exported data into your newly created database using `psql` command-line tool. This tool executes commands from standard input stream of SQL script and restores the backup file you created in the first step.


Object Mobility between tablespaces is a handy feature in PostgreSQL that enables us to manage disk space and optimize performance by moving large tables or indexes across tablespaces from one location to another. In this article, we discussed preparing source databases for object mobility, exporting data using pg_dump utility, dropping original objects, removing associated files, creating a new tablespace location and importing data into new locations.

Following our step-by-step guide will ensure safe object mobility without encountering any errors or performance degradation. By mastering these techniques of PostgreSQL database administration you can employ them effectively in your application backend designs ultimately improving your application’s overall performance.

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