Does MySQL\'s ORDER BY RAND() Function Actually Use a Random Selection Algorithm?

Is MySQL's ORDER BY RAND() Function a Random Selection Algorithm?

In the realm of database operations, retrieving data in a truly random order can be a conundrum. MySQL's ORDER BY RAND() function has long been a tool used for this purpose, but the underlying mechanism behind its apparent randomness has remained somewhat elusive.

Despite the assumption that ORDER BY RAND() adds a random column to the table before sorting, recent research has demonstrated that this is not the case. Instead, MySQL employs a different strategy to achieve its purported random selection.

In fact, the query proposed by Jay in his blog proves to be the most efficient method for obtaining a random row. This query joins the table with a subquery that generates a random ID within the table's maximum ID range. By filtering the table rows to those with IDs greater than or equal to this random ID, the query quickly retrieves a random row.

However, a peculiar observation has been made regarding the execution times of three similar queries:

• SELECT * FROM table ORDER BY RAND() LIMIT 1; (30-40 seconds)
• SELECT id FROM table ORDER BY RAND() LIMIT 1; (0.25 seconds)
• SELECT id, username FROM table ORDER BY RAND() LIMIT 1; (90 seconds)

These varying execution times seem counterintuitive, as all three queries involve sorting by a single column. The disparity can be attributed to indexing.

In the second query, the id column is indexed, allowing for rapid access to the data. However, including additional columns like username in the third query requires MySQL to read each row and allocate memory for these values, significantly increasing the execution time.

In conclusion, MySQL's ORDER BY RAND() function does not use a true random selection algorithm. Instead, it employs a technique that simulates randomness by joining the table with a subquery that generates a random ID. While not the most efficient approach, it provides a practical method for retrieving data in a seemingly random order.

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