How to Lock Non-Existent Rows in InnoDB: A Dilemma and Solutions

Locking on Non-Existent InnoDB Rows: A Technical Quandary

In the realm of database management, it may often be necessary to ensure that an operation is executed atomically, preventing any conflicts or inconsistencies. This is especially true when dealing with concurrent transactions that may attempt to modify the same data. In this article, we tackle a specific scenario where locking on a non-existent InnoDB row is desired.

The question posed is: How can one ascertain that a username does not exist in a database and then insert it as a new row without risking any interruptions between the SELECT and INSERT operations? The conventional solution involving LOCK IN SHARE MODE or FOR UPDATE, which is typically effective for existing rows, falls short in this case.

The underlying dilemma lies in MySQL's lack of a mechanism to lock non-existent records effectively. Concurrent sessions can simultaneously lock non-existent rows "FOR UPDATE," which could lead to deadlocks or duplicate key errors when attempting to insert.

To navigate this challenge, one must consider alternative approaches:

1. Semaphore Tables: This method involves creating a separate table to store semaphores, representing the non-existent rows to be locked. When a transaction initiates, it acquires a semaphore for the intended row. This effectively locks the non-existent row for the duration of the transaction, preventing concurrent insertions.
2. Table-Level Locking: An alternative solution is to lock the entire table when performing the insertion. While this approach offers a coarser level of locking, it may impact performance in scenarios where concurrent modifications occur frequently.

By understanding the limitations of MySQL's locking capabilities and employing suitable alternatives, database administrators can ensure the integrity of their data and avoid potential conflicts when dealing with non-existent rows.

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