Solving the Locking Rows Behaviour for Temporary Tables in PLPGSQL Functions

Описание: Learn how to prevent inconsistencies caused by concurrent operations in PostgreSQL using PLPGSQL functions and triggers to manage temp tables effectively.
---
This video is based on the question https://stackoverflow.com/q/78110912/ asked by the user 'msamyatl' ( https://stackoverflow.com/u/4709871/ ) and on the answer https://stackoverflow.com/a/78119330/ 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.

Visit these links for original content and any more details, such as alternate solutions, comments, revision history etc. For example, the original title of the Question was: Locking rows behaviour for a temporary table in a PLPGSQL function

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.
---
Addressing Row Locking in PLPGSQL Functions with Temporary Tables

When working with databases, especially in a concurrent environment, it is crucial to ensure the integrity of your data. This is particularly relevant when you are managing multiple tables and updates that could conflict with each other. One common issue arises when using temporary tables in PLPGSQL functions, as illustrated by our scenario involving two tables - table A (the source of truth) and table B (a denormalized version). In this post, we'll explore this problem and provide a solution.

The Problem

Background

Consider the following setup:

Table A contains a status column (a BOOLEAN) that indicates whether data is "active" or "inactive."

Table B is derived from table A, containing rows only when the status is TRUE.

As part of your database maintenance, you've created a PLPGSQL function to populate table B with existing data from table A. However, a concurrency issue arises:

A row in table A is updated from TRUE to FALSE.

Upon this update, a trigger fires and deletes the corresponding row from table B.

Meanwhile, your PLPGSQL function, which is running concurrently, still attempts to insert the now-deleted row, leading to inconsistent results in table B.

This problem highlights that the current locking mechanism (using FOR UPDATE) is not sufficient to prevent this scenario.

The Solution

1. Understanding the Trigger Issue

The key to resolving the inconsistency lies in the arrangement of your triggers. As a best practice, define your triggers before executing the PLPGSQL function that populates table B. This ensures that the deletion trigger acts on the latest data in real time before an insertion attempt can happen.

2. Incorporating Additional Triggers

In addition to handling the UPDATE, it is crucial to implement triggers for INSERT and DELETE operations. By having these triggers in place, you can ensure that any changes to the status of rows are appropriately reflected in table B.

3. Simplifying the Bulk Update Procedure

To streamline the process and prevent row locking issues, consider rewriting your bulk update operation. Instead of using a temporary table with FOR UPDATE, a more effective way is:

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

4. Why Use FOR SHARE?

The FOR SHARE lock:

Prevents concurrent modifications to the rows of table A while the insert operation is working with them.

Is less restrictive than FOR UPDATE, allowing for greater concurrency without compromising data integrity.

Conclusion

By restructuring your trigger setup and utilizing a SHARE lock during the selective insert operation, you can prevent the inconsistencies that arise from concurrent data modifications. These adjustments will create a safer and more reliable environment for managing your tables within PostgreSQL.

Taking these steps can vastly improve your database's reliability and integrity, ensuring the correct state of data in all scenarios.