How Does MySQL Determine Row Order in `SELECT * FROM table_name;` Queries?

Determining Row Order in MySQL for "SELECT * FROM table_name;"`

When executing a "SELECT * FROM table_name;" query without an ORDER BY clause, MySQL does not provide any guarantees regarding the order of the result set rows.

Dependence on Implementation Details

Unless explicitly specified using an ORDER BY clause, the row order is entirely subject to internal implementation specifics of MySQL's RDBMS engine. It may vary depending on factors such as index selection, storage layout, and optimization techniques employed by the engine.

Practical Implications

While it's possible that the rows may be returned in their original insertion order or physical storage sequence on occasion, relying on this behavior is inadvisable. This is because changing to another RDBMS or even upgrading to a newer version of MySQL could lead to a different row order.

Exceptions with InnoDB

In practice, InnoDB storage engine tends to return rows based on the order in which it retrieves them from the index used by the query optimizer. This means the order may vary depending on the specific index selected.

Example

Consider the following demonstration using InnoDB:

CREATE TABLE foo (id SERIAL PRIMARY KEY, bar CHAR(10), baz CHAR(10), KEY(bar));

INSERT INTO foo (bar, baz) VALUES
  ('test5', 'test5'), ('test5', 'test5'),
  ('test4', 'test4'), ('test4', 'test4'), 
  ('test3', 'test3'), ('test3', 'test3'), 
  ('test2', 'test2'), ('test2', 'test2'), 
  ('test1', 'test1'), ('test1', 'test1');

By default, rows are returned in primary key order (clustered index).

select * from foo;

However, using an index explicitly can change the order:

select * from foo where bar between 'test2' and 'test4';

In this case, the order is dictated by the secondary index on the bar column.

Differences Under MyISAM

Using a different storage engine like MyISAM can result in different behaviors. In MyISAM, rows are generally stored in insertion order, leading to a different default row order compared to InnoDB.

In conclusion, to ensure consistent row order in your SQL queries, always use an explicit ORDER BY clause. Relying on default ordering mechanisms can lead to unexpected results and potential data integrity issues.

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