How Can I Generate Unique Auto-Incrementing IDs for Each Unique User in MySQL?

Managing Unique Auto-Increment Keys for Different Primary Key Values

Background

A user encounters the need to create a table with a primary key (e.g., id) with auto-increment functionality and another key (e.g., uid) that references users. They seek a way to generate unique auto-increment values for id for each distinct uid value.

Solution

MySQL's MyISAM Engine

• MyISAM supports auto-increment on secondary columns in multi-column indexes.
• This generates id values as the maximum auto_increment_column value plus 1 for a given uid prefix.

Alternative Approaches

Triggers

• Cannot reliably emulate auto-increment behavior without table locking.
• Concurrent inserts may result in duplicate id values due to timing issues.

Additional Considerations

• Exclusive Table Locking: Acquiring an exclusive table lock allows for controlled inserts but may impact performance.
• Non-Transactional Solution: Keeping track of the last generated id per uid outside transaction scope can be difficult to implement and maintain.
• Unique vs. Consecutive Values: Ensure proper handling of gaps in id values due to rollbacks, failed inserts, or deletions.

Other Database Engines

• The provided solution is specific to MySQL's MyISAM engine.
• Other database engines may not support this functionality natively.

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