How Can I Achieve Multiple Unique Auto-Increment Indexes for Primary Key Values in a Relational Database?

Multiple Unique Auto-Increment Indexes for Primary Key Values

In a relational database, preserving the sequential order of auto-increment values for different primary key values can be a challenge. This article explores this functionality, discusses its availability in various database engines, and provides alternative solutions.

Native Support in MySQL MyISAM Engine

MySQL's MyISAM engine offers an elegant solution for this issue. It allows the specification of AUTO_INCREMENT on a secondary column in a multiple-column index, enabling unique auto-incrementing for each primary key value.

CREATE TABLE example (
  id INT AUTO_INCREMENT,
  uid INT,
  PRIMARY KEY (id),
  INDEX (uid)
);

In this schema, each time a new record is inserted with a given uid value, the id column for that uid will increment sequentially, regardless of other uid values.

Non-SQL Database Options

While SQL engines may not universally support this functionality, some NoSQL databases offer similar capabilities. For example:

• MongoDB: Using compound indexes with the prefix of the primary key and the unique field allows for auto-increment-like behavior.
• Cassandra: By using TimeUUID as the primary key, Cassandra automatically generates unique, sequentially increasing values.

Emulation Using Triggers

Emulating this behavior using triggers is not recommended due to potential concurrency issues. Concurrent INSERT operations can lead to primary key conflicts and data inconsistencies.

Alternative Solutions

If native support or trigger emulation is not feasible, consider the following alternatives:

• Exclusive Table Locking: Acquire an exclusive lock on the table before inserting a new record, preventing concurrent INSERTs and ensuring sequential values.
• External Id Generation: Use an external data store or service to generate unique, sequential values outside of the database transaction scope. This can prevent concurrency issues and allow for custom ordering.

Challenges and Considerations

Maintaining sequential order for different primary key values can be complex and may introduce additional challenges:

• Rollback and DELETE operations can create gaps in the sequence.
• Concurrent operations can lead to contention and reduce performance.
• Custom ordering may conflict with other requirements or constraints.

Conclusion

Various solutions exist for handling unique auto-increment indexes per primary key values. Choosing the appropriate approach depends on the specific database engine, performance requirements, and data integrity constraints.

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