Isolation Levels in Database Transactions

Isolation levels determine how transactions are managed in terms of what they can see and how they interact with other transactions happening at the same time. Each level offers a different balance between speed and keeping data accurate. Here's a look at the common isolation levels you'll find in databases like PostgreSQL and MySQL:

1. Read Uncommitted

   • Here, a transaction can peek at data that hasn't been finalized by others, which is known as a "dirty read."

   • This means you might see data that gets rolled back later, causing inconsistencies.

   • In PostgreSQL, READ UNCOMMITTED behaves like READ COMMITTED, so dirty reads aren't allowed, similar to most other databases.

Transaction 1: on MySQL

Transaction 2: On MySQL

1. Read Committed

   • Transactions only see data that's been committed, avoiding dirty reads. However, if data changes between reads in the same transaction, you might encounter "non-repeatable reads."

   • This can lead to inconsistencies if the data changes while you're still working with it.

     

2. Repeatable Read

   • This level ensures that once you read a value, it won't change during your transaction, preventing non-repeatable reads.

   • However, it still allows "phantom reads," where new rows added or removed by others can change your query results even within the same transaction.

     

3. Serializable

   • This is the strictest level, making transactions run as if they were one after the other, without overlap.

   • In MySQL: Transactions might get blocked if they conflict with others, which can slow things down but keeps data consistent.

   • In PostgreSQL: It uses Serializable Snapshot Isolation (SSI), which keeps things consistent without blocking reads. Transactions can run side by side but still appear as if they're in order.

     • Snapshot Isolation: Each transaction sees a snapshot of the database from when it started, ignoring changes made by others afterward.

     • Conflicts are caught when you try to commit. If two transactions clash over the same data, PostgreSQL will stop one to keep things orderly.

Key Differences: MySQL vs PostgreSQL Serializable

• MySQL: With SERIALIZABLE, reads can get blocked if there's a conflict.

• PostgreSQL: Uses SSI to avoid blocking reads, catching conflicts when you commit instead.

Conflict Handling in PostgreSQL:

• If two transactions try to change the same data, PostgreSQL will catch this when you commit. The second one gets stopped, so its changes don't go through, as if it never happened.

This way, PostgreSQL keeps things consistent without stopping reads, unlike MySQL, which might block them in these cases.