Does MySQL Guarantee Row Order Without an ORDER BY Clause?

MySQL Row Order for "SELECT * FROM table_name;": An Uncertain Fate

Despite the common misconception, MySQL does not guarantee the order of rows returned by the "SELECT * FROM table_name;" query without an ORDER BY clause. This seemingly simple query relies heavily on the internal implementation of the database engine, making the order unpredictable.

Typically, the rows are returned based on the order in which they are stored on the disk. However, the storage mechanism can vary among different MySQL storage engines, such as InnoDB and MyISAM, which can influence the returned order.

In InnoDB, the rows are typically retrieved based on the index that is chosen by the optimizer for the query. This could lead to a different order compared to the primary key order or the insertion order. In MyISAM, rows are stored in the order of their creation, resulting in a somewhat predictable order for rows inserted one after another.

However, it's crucial to emphasize that relying on this default order without using an ORDER BY clause is risky. Any changes in storage implementation or database version could disrupt the assumed order, potentially leading to inconsistencies.

For instance, deleting and then reinserting rows can disrupt the original insertion order in MyISAM. Upgrading to a newer MySQL version that uses a different storage architecture could also alter the row order in previously created tables.

To ensure a consistent row order, it is highly recommended to explicitly specify the ordering criteria using an ORDER BY clause. This approach guarantees the desired row order regardless of the internal implementation or database version.

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