Скачать или смотреть Resolving the cursor does not exist Error in PL/pgSQL

Learn how to fix the `cursor does not exist` error in PostgreSQL with practical coding solutions and better practices for loading CSV files.
---
This video is based on the question https://stackoverflow.com/q/67474133/ asked by the user 'sev' ( https://stackoverflow.com/u/4923949/ ) and on the answer https://stackoverflow.com/a/67474639/ 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, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: ERROR cursor does not exist after first loop in PL/pgSQL

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.
---
Resolving the cursor does not exist Error in PL/pgSQL

If you're working with PostgreSQL and diving into PL/pgSQL for tasks like importing CSV files, you might encounter a frustrating error: cursor does not exist after the first loop. This common issue can halt your data loading process, especially when dealing with large datasets. In this guide, we will explore why this error occurs and how to effectively resolve it, so you can streamline your data management tasks without further interruptions.

Understanding the Problem

You are aiming to load a large number of CSV files into a PostgreSQL database. The setup includes a table called source_files that tracks file paths and whether they have been successfully loaded into the database. In the given situation, your cursor correctly handles the first file but raises the error on subsequent iterations. This typically occurs when the cursor's transaction scope is violated during execution.

The Core Issue: Cursor Behavior and Transactions

The main culprit behind the cursor does not exist error in your code is the use of the COMMIT statement within the DO block. Here’s the crux of the problem:

When you call COMMIT, the cursor associated with that transaction is closed, making it unavailable for subsequent loops.

PL/pgSQL does not support cursors that remain valid after a transaction has ended, which means that once you commit, the cursor gets discarded.

Recommended Solution

To resolve this issue, we recommend the following actions:

Remove the COMMIT Statement: Since you are using a DO statement, it’s best to handle all operations within a single transaction without committing until all entries are processed.

Revising Your Code

Along with removing COMMIT, we can streamline your code for better clarity and security. Here are the steps to update your PL/pgSQL block:

1. Use Implicit Cursor in Loop

Instead of manually managing a cursor, consider using an implicit cursor. This approach eliminates the need for cursor declarations and streamline your loop structure. Here’s how you can implement it:

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

2. Secure Your SQL Command

It's crucial to prevent SQL injection vulnerabilities, especially when forming SQL commands dynamically. Instead of using %s in the FORMAT function, use the %L format specifier, which safely escapes input:

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

Simplifying CSV Loading Logic

Keep the logic for loading CSV and processing records within the loop. You can maintain the steps related to creating temporary tables, importing data, and updating the source_files table as previously established, but within the simpler implicit cursor structure.

Sample Updated Code Section

Here’s a quick outline of how your revised code might look after applying these recommendations:

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

Conclusion

By removing the COMMIT statement and leveraging implicit cursors while securing your SQL commands, you can eliminate the dreaded cursor does not exist error and enhance the reliability of your data loading process. The updated approach not only simplifies your code but also enhances its security, ensuring a smoother experience when importing data into PostgreSQL.

Implement these strategies in your database tasks and see a significant reduction in errors and code complexity. Happy coding!