Why Is Using a Subquery in the WHERE Clause So Slow in MySQL?

MySQL Performance Impact: Subquery in WHERE Clause

When dealing with duplicate data, it's necessary to identify and inspect the offending rows. However, a straightforward query using a subquery in the WHERE clause can encounter significant performance issues.

The Original Query:

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

Despite the index on relevant_field, this query performs exceptionally slowly. The reason lies in the subquery's correlated nature.

Correlated Subqueries and Performance:

A correlated subquery is one that references columns from the outer query. In this case, the subquery selects the relevant_field values that occur more than once in the table. For each row in the outer query, the subquery is executed, leading to multiple executions of the same query. This results in poor performance.

Non-Correlated Subqueries as a Solution:

To eliminate the performance issue, it's recommended to transform the correlated subquery into a non-correlated one. This can be achieved by selecting all columns from the subquery and assigning it an alias.

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

The Modified Query:

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 query performs much faster because the subquery is no longer correlated. It calculates the relevant_field values that occur more than once in a single execution and stores them in the subquery alias, which is then used in the main query.

Conclusion:

Understanding the difference between correlated and non-correlated subqueries is crucial for optimizing MySQL performance. By transforming correlated subqueries into non-correlated ones, you can significantly improve the speed of queries that rely on subqueries in the WHERE clause.

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