Transitioning from Oracle to PostgreSQL: Understanding the Concept of Schema

As businesses increasingly move toward open-source technologies, many Oracle Database professionals find themselves needing to work with PostgreSQL, one of the most popular open-source relational database management systems (RDBMS). Although both Oracle and PostgreSQL share many similar concepts, there are fundamental differences in how these systems handle certain database structures, one of which is the schema.

In Oracle, the term “schema” is tightly intertwined with the concept of users, but in PostgreSQL, the schema plays a somewhat different role. For Oracle users accustomed to one-to-one relationships between users and schemas, this can be a source of confusion. In this article, we’ll dive deep into the differences in the use of schemas between Oracle and PostgreSQL, explaining how they are structured, used, and how you can leverage PostgreSQL schemas effectively in your database management practices.

Introduction: What is a Schema?

Before we explore the differences between Oracle and PostgreSQL, let’s briefly define what a schema is in general terms. In database systems, a schema typically refers to a collection of database objects such as tables, views, indexes, and procedures. A schema provides a namespace for these objects, ensuring that each object is uniquely identifiable within that space.

However, the concept of a schema can vary depending on the RDBMS being used. In both Oracle and PostgreSQL, schemas provide a mechanism to organize objects, but the way they interact with users, access control, and object management differs significantly.

Let’s begin with a review of how schemas work in Oracle, and then look at how PostgreSQL handles schemas differently.

Oracle’s Approach to Schemas: A User-Centric Model

Schema is Synonymous with User

In Oracle, a schema is essentially equivalent to a user account. When a user is created in Oracle, a schema is automatically created for that user. The schema contains all the objects (tables, views, indexes, etc.) owned by that user. Thus, there is a one-to-one relationship between users and schemas in Oracle databases.

This structure means that each user has their own workspace within the database, and any objects they create are stored within their own schema. This tight coupling of users and schemas simplifies access control, as only the user who owns the schema can access and modify objects in that schema, unless explicit permissions are granted to other users.

For example, if a user named SCOTT owns a table called EMPLOYEES, the fully qualified name of that table in Oracle would be SCOTT.EMPLOYEES. This naming convention clearly identifies both the user and the object they own.

User-Schema Separation: A Key Limitation

The user-schema relationship in Oracle presents a few limitations, especially in environments where multiple users need to work with the same set of database objects. Since each user has their own schema, sharing objects between users can become cumbersome. For instance, if multiple users need access to the SCOTT.EMPLOYEES table, each user must either be granted explicit permissions or use synonyms to reference the object without including the schema in the query.

This limitation can lead to more complex access control mechanisms and object management as the number of users and shared objects increases. Furthermore, schema design and database organization in Oracle tend to revolve around user accounts, making it less flexible for larger, collaborative environments where objects may not belong to a specific user but to a department, team, or application.

PostgreSQL’s Approach to Schemas: A Flexible Namespace Model

Schemas as Namespaces

In PostgreSQL, schemas serve as logical namespaces within a database, and they are not tied directly to any specific user. This distinction is important: unlike in Oracle, where each user owns a schema, PostgreSQL allows you to create multiple schemas within the same database, and these schemas can contain objects owned by different users.

This flexible approach to schemas in PostgreSQL provides a level of organization that is more scalable and modular. Schemas in PostgreSQL can be thought of as folders or containers that group related objects together. This makes it easier to organize objects by function, department, or application within the same database.

For example, consider a PostgreSQL database for a human resources application. You might have a schema called hr that contains tables like employees, departments, and salaries. The fully qualified name of the employees table would be hr.employees. Multiple users can access this schema, and the objects within it can be owned by different users.

Decoupling Users and Schemas

Unlike Oracle’s strict coupling between users and schemas, PostgreSQL allows for greater flexibility. A user can own objects across multiple schemas, and a schema can contain objects owned by different users. This decoupling provides more granular control over database organization and access, which can be especially beneficial in environments with many users and applications.

For instance, in PostgreSQL, you could create a schema for each department in your organization (e.g., hr, finance, sales), and within each schema, you can have objects owned by different users. This provides a cleaner structure and allows users to work within the same database without the need to cross schema boundaries frequently or rely on schema-specific permissions.

PostgreSQL also allows you to control the search path, which determines the order in which schemas are searched when an object is referenced without a schema prefix. By configuring the search path, you can simplify queries by allowing users to access objects without always specifying the schema name.

Key Differences Between Oracle and PostgreSQL Schemas

Now that we’ve established the basic concepts of schemas in Oracle and PostgreSQL, let’s summarize the key differences in how each system handles schemas:

1. Relationship Between Schemas and Users

Oracle

A schema is directly associated with a user account, and there is a one-to-one relationship between the user and their schema. The schema essentially represents the user’s workspace in the database.

PostgreSQL

Schemas are independent of users. Multiple users can own objects within the same schema, and a single user can own objects across multiple schemas. This allows for a more flexible and modular approach to database organization.

2. Schema Creation

Oracle

Schemas are created automatically when a user account is created. You don’t explicitly create a schema in Oracle; instead, you create a user, and the schema is created for that user.

PostgreSQL

Schemas are created explicitly using the `CREATE SCHEMA` command. You can create multiple schemas within a single database, and these schemas can be used to organize objects logically without regard to the users who own the objects.

3. Object Organization

Oracle

Objects are organized within the schema associated with the user who created them. Each user’s objects are stored in their own schema, which can make sharing objects between users more complex.

PostgreSQL

Objects are organized within schemas, and these schemas are not tied to individual users. Multiple users can share the same schema, making it easier to organize objects by function or application rather than by user.

4. Access Control

Oracle

Since schemas are tied to users, access control is closely related to user privileges. If a user needs to access objects in another user’s schema, permissions must be granted explicitly, or synonyms must be used to simplify object access.

PostgreSQL

Access control is more flexible, as multiple users can own and share objects within the same schema. Permissions can be granted at the schema level, allowing users to access objects without needing to reference the schema name in every query.

5. Fully Qualified Object Names

Oracle

Objects are referenced using the format `schema_name.object_name`. Since each schema corresponds to a user, this means that objects are typically referenced as `user_name.object_name`.

PostgreSQL

Objects are referenced using the format `schema_name.object_name`, but since schemas are independent of users, the schema name reflects the logical organization of the database rather than the user who owns the object.

6. Schema Management in Large Environments

Oracle

Managing schemas in large environments can be more complex due to the one-to-one relationship between users and schemas. This structure often requires more granular access control mechanisms and can make object sharing more cumbersome.

PostgreSQL

The decoupling of schemas and users makes it easier to manage large environments. Schemas can be used to group related objects, and users can access objects across multiple schemas without the need for complex access control mechanisms.

Practical Considerations: Moving from Oracle to PostgreSQL

For Oracle professionals moving to PostgreSQL, the most significant adjustment is likely the decoupling of users and schemas. In Oracle, the user-centric approach to schemas means that database organization revolves around individual user accounts. In PostgreSQL, however, the schema becomes a more flexible organizational tool, independent of users.

Here are a few practical tips for transitioning from Oracle’s schema model to PostgreSQL:

1. Rethink Database Organization

In Oracle, you might be used to creating a new user for each application or department and relying on the one-to-one relationship between users and schemas to organize objects. In PostgreSQL, you should think about creating schemas based on functionality, application modules, or organizational departments, rather than individual users.

For example, instead of creating a separate user for the HR department, create an hr schema and organize all related objects (e.g., tables, views, functions) within that schema. Multiple users can then be granted access to this schema as needed.

2. Leverage the Search Path

PostgreSQL allows you to configure the search path, which determines how the system resolves object names that are not fully qualified (i.e. when you don’t specify the schema). By setting the search path appropriately, you can simplify your queries and avoid the need to specify schema names constantly.

For instance, if most of your work is done in the hr schema, you can set the search path to include hr first, so you can reference tables like employees without needing to prefix the schema name in every query.

3. Use Roles for Access Control

In PostgreSQL, roles (which can represent either a user or a group of users) are used to manage access to objects across schemas. By using roles effectively, you can control who has access to which schemas and what actions they can perform on the objects within those schemas.

For example, you might create a read_only role that has permission to SELECT from tables in the hr schema, and then assign this role to users who need read-only access to HR data.

4. Plan for Schema Migration

If you’re migrating an Oracle database to PostgreSQL, you’ll need to carefully plan how to map Oracle users and schemas to PostgreSQL schemas. Since Oracle ties schemas directly to users, you may end up with many schemas in PostgreSQL if you follow the same structure

Instead, consider consolidating related objects into fewer schemas based on their logical relationships rather than their user ownership. This can help simplify your database structure and make it easier to manage in the long term.

Conclusion: Embracing PostgreSQL’s Flexibility

Transitioning from Oracle to PostgreSQL requires a shift in how you think about schemas and database organization. In Oracle, schemas are tightly coupled with users, creating a straightforward but sometimes restrictive environment. PostgreSQL, on the other hand, decouples schemas from users, providing a more flexible and scalable system for organizing and managing database objects.

By understanding the differences in how schemas work in Oracle versus PostgreSQL, you can make more informed decisions about how to structure your databases and manage access control in PostgreSQL. With its flexible namespace model and powerful schema management tools, PostgreSQL offers a robust platform for organizing complex databases in a way that promotes collaboration and scalability.