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Learn how to effectively grant privileges on tables in PostgreSQL databases while understanding the limitations of cross-database references.
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This video is based on the question https://stackoverflow.com/q/66916694/ asked by the user 'LeGEC' ( https://stackoverflow.com/u/86072/ ) and on the answer https://stackoverflow.com/a/66917276/ provided by the user 'Laurenz Albe' ( https://stackoverflow.com/u/6464308/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Granting Privileges in PostgreSQL: Navigating Cross-Database Limitations

When working with PostgreSQL databases, managing user privileges efficiently is crucial for maintaining security and proper access control. A common scenario arises when you want to grant specific table permissions to a user but are concerned about doing so on the wrong database. In this post, I'll address a common question regarding granting privileges in PostgreSQL, especially when dealing with multiple databases.

The Challenge

Many database administrators find themselves in situations where they want to provide users with certain privileges on tables, particularly when managing a public schema in a specific database. For example, you might be connected to a database, let’s say mydb, and want to grant user myuser permissions to insert, update, delete, and truncate tables within the public schema.

However, what if you want to specify the database in your SQL query rather than relying solely on the current database connection? Here’s the context shared by one user facing this challenge:

User Connection: They connect as user postgres to the mydb database.

SQL Command Attempt: They attempt to run the query:

[[See Video to Reveal this Text or Code Snippet]]

Concern: They want to be sure they are not unintentionally giving privileges to the postgres database instead of mydb.

The underlying question this user posed was straightforward: Is there a way to target tables in another database when constructing a GRANT query in PostgreSQL?

The Answer

The sad reality is: No, you cannot grant privileges in another database directly through a typical SQL GRANT command. The error message that surfaces when trying to do so states, "cross-database references are not implemented." Let’s breakdown what this means:

Security Feature

Single Database Access: PostgreSQL is designed with a security feature that prevents users from affecting databases other than the one they are connected to.

Isolation of Databases: This is crucial for maintaining the security and integrity of each database, ensuring that actions in one database do not inadvertently impact another.

Alternative Solutions

Although the GRANT command cannot be used across databases directly, there are a few approaches you might consider if you need to manage privileges effectively:

Connect to the Target Database:

Simply connect to the database where you want to grant privileges and run the appropriate GRANT command.

Use dblink or Foreign Data Wrappers:

PostgreSQL does allow for certain operations across databases using extensions such as dblink or foreign data wrappers. However, these methods come with their own complexities and are best reserved for advanced use cases.

Conclusion

While it may feel limiting not to be able to specify a different database in your GRANT commands, this restriction is ultimately a design choice born from a focus on security. Ensuring that database operations are performed strictly within the context of the connected database helps prevent unintended consequences.

If you need to manage user privileges effectively, ensure you're connected to the right database before executing commands, and consider using extensions like dblink for cross-database interactions when absolutely necessary.

Understanding these nuances can greatly assist in maintaining your PostgreSQL databases while upholding their security and integrity.