How Can I Achieve Separate Auto-Increment Sequences Based on Specific Primary Key Values in a Database?

Multiple Auto-Increments for Specific Primary Key Values

Question:

How can a database provide different auto-increment keys for specific primary key values, so that each primary key value has its own independent sequence? For example, if a table contains an "id" column with auto-increment and a "uid" column for user ID, is it possible to have a different auto-increment sequence for each "uid" value?

Answer:

MySQL with MyISAM Engine

MySQL's MyISAM engine supports this functionality. When AUTO_INCREMENT is specified on a secondary column in a multiple-column index, the generated value for the AUTO_INCREMENT column is calculated as MAX(auto_increment_column) 1 WHERE prefix=given-prefix.

Other Solutions

Exclusive Table Lock

One alternative solution is to acquire an exclusive table lock before attempting an INSERT. This prevents concurrent INSERTs from creating race conditions. However, it serializes access to the table, limiting throughput.

External Id Generation

Another approach is to generate the id number outside of transaction scope, using mechanisms like memcached or a centralized service. This ensures unique id values without relying on database functionality.

Considerations

Implementing this functionality requires careful consideration of potential issues, such as:

• Rollbacks and gaps in auto-increment sequences
• Failed INSERTs due to other table constraints
• Renumbering after row deletions
• Maintaining consistency in case of external id generation

Disclaimer:

The InnoDB engine in MySQL does not support this feature.

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