Understanding PostgreSQL Transactions: Why SELECT Queries Wait During Long INSERTs

Описание: A deep dive into why SELECT queries are blocked during long-lasting transactions in PostgreSQL and how to manage table locks effectively.
---
This video is based on the question https://stackoverflow.com/q/65347930/ asked by the user 'Daniel San' ( https://stackoverflow.com/u/1827242/ ) and on the answer https://stackoverflow.com/a/65353643/ 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: Can not execute select queries while making a long lasting insert transaction

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.
---
Trouble with SELECT Queries During Long Transactions in PostgreSQL

If you've ever worked with PostgreSQL, you might have encountered a frustrating situation: you're running long-lasting INSERT transactions, and suddenly, your SELECT queries start to hang. This can be particularly confusing for those new to PostgreSQL. Let's explore why this happens and how you can manage long transactions more effectively.

The Cosmos of Database Transactions

In PostgreSQL, transactions are crucial for maintaining data integrity. When you execute a transaction, you essentially create a set of changes that either all occur or none at all. This means that different operations within the same transaction may lock certain resources, which can affect how other operations, like SELECT, behave.

The Problem Explained

In the scenario presented, a user is running a big DROP TABLE and INSERT transaction that takes a considerable amount of time—up to 30 minutes. During this long transaction, they attempt to execute a SELECT query, only to find that the query execution waits until the transaction is finished.

Why Does This Happen?

Access Exclusive Lock: When you issue a command like DROP TABLE, PostgreSQL places what is called an ACCESS EXCLUSIVE lock on that table. This lock prevents any other operations from modifying or even reading from that table while the DROP TABLE command is pending.

Transaction Scope: PostgreSQL holds locks until the entire transaction is complete. This means that until the long INSERT operation finishes, the table remains inaccessible to any other queries.

Concurrency Constraints: Allowing concurrent access to a table during a drastic operation like DROP TABLE could lead to data inconsistency or system errors. Imagine a situation where a COMMIT is blocked by a concurrent SELECT, or a SELECT crashes while trying to read from a changing structure.

Solutions to Manage Long Transactions

Understanding the locking mechanism can help you navigate long transactions more effectively. Here are some strategies:

1. Avoid DROPPING Tables During Transactions

If possible, avoid dropping tables while conducting long operations. Instead of dropping and recreating the table, consider the following:

Truncate the Table: If you only need to empty the table, TRUNCATE can be used, which is considerably faster and doesn’t require an ACCESS EXCLUSIVE lock unless there are foreign key constraints.

Rename Tables: Instead of dropping a table, you might rename it (e.g., ALTER TABLE old_table RENAME TO new_table) and then create a new table with the original name.

2. Batch Inserts

Instead of a single long transaction for inserts, consider breaking them into smaller batches. This can reduce the time locks are held and improve overall responsiveness:

Commit in Chunks: Instead of waiting for a long operation to complete, commit smaller sets of data regularly. This approach not only mitigates locking but also keeps the database responsive.

3. Maintain Readability

If there are long running operations, it’s often best to keep the data in a readable state. By considering alternate methods to modify table data without dropping tables, you can ensure users querying the table won’t experience long waits.

Conclusion

By understanding how PostgreSQL handles locks during transactions, especially with operations like DROP TABLE, you can create a more efficient workflow that minimizes downtime for SELECT queries. Keeping tables intact and focusing on strategies to batch operations can significantly improve the user experience and data accessibility during lengthy transactions.

Navigating database transactions effectively requires a proactive strategy—always consider the implications of