Why is my MySQL subquery in an IN clause causing performance issues?

MySQL - Subquery Correlation Slowing Down IN Clause Performance

The given query, which identifies duplicate rows in a database table using a subquery in the IN clause, is experiencing significant performance issues. Although the subquery itself executes quickly, the IN clause query takes considerably longer.

Reason for Slowness

The slowdown is caused by the correlated nature of the subquery. In a correlated query, the subquery references columns from the outer query, making it dependent on the outer query's execution. As a result, each row in the outer table triggers a separate execution of the subquery.

Solution

To resolve the issue, the correlated subquery can be converted into a non-correlated subquery by selecting all columns in it. This eliminates the dependency on the outer query, significantly improving performance:

SELECT * FROM
(
    SELECT relevant_field
    FROM some_table
    GROUP BY relevant_field
    HAVING COUNT(*) > 1
) AS subquery

Final Query

The modified query will look like this:

SELECT *
FROM some_table
WHERE relevant_field IN
(
    SELECT * FROM
    (
        SELECT relevant_field
        FROM some_table
        GROUP BY relevant_field
        HAVING COUNT(*) > 1
    ) AS subquery
)

This non-correlated query will execute much faster than the original correlated query.

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