intro
Let’s learn everything you need to know about the concept of a database catalog, which is fundamental to understanding how the metadata of a database is stored.
There are some key SQL and database concepts that every developer, system administrator, and database user should understand to better grasp how a database engine works. One such concept is the database catalog.
In this guide, you'll learn everything about it, including why the term is often confused with other similar terms commonly used in the world of SQL.
Let’s dive in!
Database Catalog: Definition in SQL
In SQL, a database catalog is a centralized metadata repository that stores detailed information about all objects within a database system. This includes metadata about tables, views, indexes, stored procedures, temporary tables, users, and their permissions.
Keep in mind that each relational database has its own catalog. This serves as a single source of truth for the DBMS to query and access all metadata about the database's structure. In other words, it plays a fundamental role in query planning, validation, and ensuring integrity and consistency across database operations.
How to Access the Database Catalog
According to the SQL standard, the database catalog should be accessible through the INFORMATION_SCHEMA system views—as it happens in systems like SQL Server, MySQL, and PostgreSQL. However, not all databases fully adhere to the standard. Some implement alternative methods to expose catalog information or may restrict access to it entirely.
Now, see how to access the database catalog in MySQL, PostgreSQL, SQL Server, and Oracle.
MySQL
In MySQL, INFORMATION_SCHEMA is a special database that provides access to metadata. This includes details such as database and table names, column data types, and user privileges. That kind of metadata is also commonly referred to as the “data dictionary” or the “system catalog.”
You can query it directly by running SQL statements against the information_schema database on your MySQL server:
PostgreSQL
In PostgreSQL, the information_schema consists of a collection of views that provide metadata about the objects defined in the current database—such as tables, columns, and privileges. It strictly follows the SQL standard, making it portable and stable across different database systems.
You can explore the information_schema within any PostgreSQL database. The most useful metadata is found in its standard-compliant views:
Note: PostgreSQL-specific features are not covered by information_schema. For those, query the system catalogs or PostgreSQL-specific views. These are implementation-specific and tailored to PostgreSQL internals.
SQL Server
The INFORMATION_SCHEMA views in SQL Server follow the ISO SQL standard for exposing metadata. They offer a system-table-independent way to query information about database objects such as tables, columns, and views. These views are especially useful for maintaining compatibility across SQL Server versions, even when underlying system tables change.
You can access these views through the INFORMATION_SCHEMA schema within your SQL Server database:
Oracle
Oracle does not provide INFORMATION_SCHEMA views. Instead, it uses its own data dictionary. This is a collection of read-only tables and views that store metadata about the database. These are often referred to as “static” because they are updated only when structural changes occur in the database, such as creating a table or granting privileges to a user.
Database Catalog vs Schema vs Database
Online and even among experienced database administrators, there is often confusion between the terms "database catalog," "schema," and "database." Now that you understand what a database catalog is, you might be wondering how it relates to schemas and databases, as these are quite different concepts.
Well, the confusion arises because some databases use the term “catalog” to describe a set of schemas within a server. To clarify, let’s first define what schemas and databases are:
To make this clearer, below is a comparison table between the terms “database,” “schema,” “catalog,” and “database catalog”:
| Term | Description | Note |
|---|---|---|
| Database | A collection of database objects, including tables, views, etc. | Can have multiple schemas. |
| Schema | A logical grouping of database objects. | Can be grouped by a database. |
| Catalog | A set of schemas | Supported only by some database engines. |
| Database Catalog | A set of views and tables to describe the entire metadata system in a database. | Typically refers to metadata storage. |
Conclusion
In this guide, you learned what a database catalog is, what kind of information it contains, and how to access it across major database systems. Whether you are working with MySQL, PostgreSQL, SQL Server, Oracle, or over 50 other databases, DbVisualizer has you covered.
As shown here, DbVisualizer lets you visually manage your data, including access to advanced tables like INFORMATION_SCHEMA. This is just one of the many powerful features of this database client—others are SQL formatting, ERD-style schema generation, query optimization, and more. Download DbVisualizer for free today!
FAQ
What is the alternative meaning of catalog in SQL?
In the context of databases, the term "catalog" can also refer to a collection of schemas. A catalog in this sense is a logical grouping or a higher-level structure that contains schemas, which in turn contain database objects like tables, views, and indexes. Some databases, like Oracle, use catalogs to describe a set of schemas that help organize objects within the system.
What is the relationship between a database catalog and INFORMATION_SCHEMA?
The INFORMATION_SCHEMA is a standardized way defined by SQL standard to access the metadata information in the database catalog.
Which database technologies follow the database catalog standard specification?
Several relational database management systems adhere to the database catalog standard specification, including MySQL, SQL Server, and PostgreSQL. These systems follow the SQL standard for accessing metadata via the INFORMATION_SCHEMA views and tables.
What are the differences between standard INFORMATION_SCHEMA views and system views?
The main difference between standard INFORMATION_SCHEMA views and system views lies in their adherence to SQL standards and their content. INFORMATION_SCHEMA views follow the SQL standard and provide a portable, abstracted way to access metadata across different databases. On the other hand, system views are specific to the database system and offer more detailed, often implementation-specific information.
What is the difference between the system catalog, database catalog, and data dictionary?
The three concepts are similar and can sometimes refer to the same thing, depending on the database being used. Specifically:
