Скачать или смотреть Understanding the suppress_redundant_updates_trigger() Issue in PostgreSQL

Discover the nuances behind the unexpected behavior of `suppress_redundant_updates_trigger()` in PostgreSQL with practical insights and solutions.
---
This video is based on the question https://stackoverflow.com/q/63815250/ asked by the user 'Stephane Desnault' ( https://stackoverflow.com/u/7085345/ ) and on the answer https://stackoverflow.com/a/63815618/ provided by the user 'jjanes' ( https://stackoverflow.com/u/1721239/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: A bug in PostgreSQL suppress_redundant_updates_trigger?

Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/l...
The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license.

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Understanding the suppress_redundant_updates_trigger() Issue in PostgreSQL

When working with triggers in PostgreSQL, developers may occasionally encounter unexpected behavior that can be quite perplexing. One such instance relates to the built-in function suppress_redundant_updates_trigger(). In this guide, we’ll explore a specific scenario where this trigger doesn't behave as expected, and we’ll break down the technical reasons behind it.

The Problem

Recently, a user reported a puzzling situation while configuring triggers for a PostgreSQL table. The user created a table and applied two "before each row" triggers. Here’s a simplified overview of how the problem emerged:

Trigger Setup: The user established a table and defined two triggers.

First Update: When attempting to update a row with conditions that were met (UPDATE test SET id = 1 WHERE id = 1), the first trigger, suppress_redundant_updates_trigger(), worked as intended, preventing the update and suppressing any further action (like firing the second trigger).

Unexpected Behavior: After altering the table to add a new column, the same update command did not suppress the update, leading to unexpected notifications from the second trigger.

This raised the critical question: Is this unexpected behavior part of how PostgreSQL operates, or is it a bug?

The Solution

Understanding the Behavior

The key issue stems from how PostgreSQL compares the old and new records during the update process. Here’s what happens under the hood:

NULL Representation: When a new column (e.g., newcol) is added to the table, it is initialized to NULL for existing records. PostgreSQL treats the representation of a record with a NULL value differently.

Tuple Comparison Mechanics: PostgreSQL does not compare records field by field. Instead, it performs a total memory comparison using a function like memcmp. This means the records are considered unequal if there are any differences at all—even if they are "invisible" to the user, like a newly added NULL value.

Intentional Design Choices: This method is designed for efficiency and simplicity, which can lead to such behaviors during updates. Therefore, the incident is not likely a bug but rather an intentional aspect of how PostgreSQL manages triggers.

Key Takeaways

Memory Comparison: Always remember that PostgreSQL uses memory comparison (like memcmp) for row updates, which considers even minor discrepancies significant—such as the presence of a new column with NULL values.

Trigger Logic: The execution of triggers can be influenced by changes in table structure, emphasizing the need to be aware of any alterations to the database schema that might affect trigger behavior.

Testing Behavior: To ensure reliable behavior of your triggers, consider testing updates under various scenarios after making schema changes.

Conclusion

Encounters with unexpected behavior in SQL databases can be frustrating, especially when triggers are involved. Understanding that suppress_redundant_updates_trigger() relies on memory comparisons helps clarify why certain updates might not be suppressed as expected following schema alterations. Keep this in mind as you work with PostgreSQL, and always test your triggers after making structural changes to your tables.

By grasping these nuances, you can better navigate PostgreSQL’s capabilities and limitations, ensuring that your data integrity remains intact as you implement your workflows.