How is Row Order Determined in a MySQL `SELECT *` Query Without `ORDER BY`?

Determining Row Order in MySQL's "SELECT * FROM table_name;" Query

When executing a simple "SELECT * FROM table_name;" query in MySQL, the order in which the result set rows appear is not guaranteed by default. This is because MySQL's internal implementation determines the row order based on various factors, including:

No ORDER BY Clause

In the absence of an explicit ORDER BY clause, MySQL does not provide any specific guarantees regarding row order. The rows may appear in:

• Insertion order (the sequence in which the rows were initially inserted)
• Physical storage order (the arrangement of the rows in the underlying storage system)
• Other optimizations selected by the database engine

Implementation Details

Therefore, relying on row order in the absence of an ORDER BY clause is not a reliable practice. Different versions of MySQL or even different RDBMS implementations may handle row ordering differently.

InnoDB Considerations

In InnoDB storage engines, default behavior typically returns rows in the order they are read from an index. However, this order can change depending on the index used by the optimizer and the specific query conditions.

MyISAM Considerations

MyISAM storage engines, on the other hand, generally store rows in the order they were inserted. However, deletion operations can create gaps in table storage, leading to newly inserted rows being placed in these gaps. This can result in row ordering discrepancies from the original insertion sequence.

Conclusion

To achieve a specific row order in your MySQL queries, it is essential to use an ORDER BY clause to explicitly define the desired sorting criteria. Relying on implicit ordering mechanisms can lead to unpredictable results and should be avoided for reliable applications.

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