Скачать или смотреть How to Create a Trigger in Postgres That Works Across Multiple Schemas

Discover how to effectively create triggers in Postgres that function seamlessly in different schemas without hardcoding schema names.
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This video is based on the question https://stackoverflow.com/q/63216348/ asked by the user 'craigmiller160' ( https://stackoverflow.com/u/2223059/ ) and on the answer https://stackoverflow.com/a/63217175/ provided by the user 'Mike Organek' ( https://stackoverflow.com/u/13808319/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Introduction

When developing applications using PostgreSQL, encountering issues related to triggers, especially in multi-schema setups, is not uncommon. One common problem developers face is the functionality of a trigger that works perfectly in a specific schema but fails during actual application writes. This post will help you understand the issue at hand and explain how to resolve it without complicating your code base by hardcoding schema names.

The Problem

In our scenario, we have a trigger meant to validate client_id, user_id, and role_id within a table called client_user_roles. The trigger is correctly set up to raise exceptions if any required fields are NULL. However, developers noticed that when trying to execute this logic from different schemas (like dev, qa, and prod), the trigger fails to locate the required tables (roles and client_users).

The issue arises because the application queries the database without being locked to the schema from which the trigger was set. As a result, the trigger can't find the tables it needs to reference, leading to failure.

The Solution

Fortunately, there's a neat way to solve this issue using PostgreSQL's built-in functionality: the TG_TABLE_SCHEMA variable. This allows the trigger to dynamically reference the schema in which it is being invoked.

Step-by-Step Explanation

Here's a breakdown of the solution, which involves making a small adjustment to the existing trigger function.

1. Use TG_TABLE_SCHEMA

The key to solving this problem is to use the TG_TABLE_SCHEMA variable. This variable contains the schema name of the table that invoked the trigger. By configuring the search path inside the trigger function, you can ensure that it correctly references the needed tables based on the current schema.

2. Adjust the Trigger Function Code

To implement this, we need to modify the validate_client_user_role() trigger function slightly:

Updated Function Code

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

3. Why This Works?

By calling PERFORM set_config('search_path', TG_TABLE_SCHEMA, true);, you are dynamically setting the search path to the current schema where the trigger is invoked. This means that when your application writes to a table in any schema, the trigger will now correctly reference the associated roles and client_users tables without needing any code modifications specific to each schema.

Conclusion

Creating an effective trigger in PostgreSQL that works across multiple schemas doesn’t have to be a complex task. By utilizing TG_TABLE_SCHEMA, you can write a single trigger function that is flexible enough to adapt to various schemas. This way, you maintain clean, efficient code without duplicating efforts across your development, QA, and production environments.

With this approach, you can develop more maintainable applications and avoid cumbersome schema-specific adjustments in the future. Embrace the power of PostgreSQL to streamline your development process!