Why does using `WHERE IN (Subquery)` lead to performance issues in MySQL when searching for duplicate rows?

MySQL - SELECT WHERE Field IN (Subquery) Performance Drop

In a database, detecting duplicate rows using a subquery can be an efficient approach. However, when attempting to retrieve all rows with duplicate field values, the WHERE IN (subquery) query might experience significant performance issues. Understanding the reason for this slowdown can help optimize the query.

Correlated Subqueries and their Impact

The reason for the performance drop in this scenario lies in the use of correlated subqueries. A correlated subquery references values from its parent query, which means the subquery is executed once for each row being processed in the parent query. In the provided query:

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

The subquery returns duplicate field values in some_table. When the outer query processes each row in some_table, it compares the relevant_field value to the values returned by the subquery. Since the subquery returns multiple rows for each duplicate field value, this comparison involves executing the subquery multiple times, leading to extended execution times.

Addressing Correlated Subqueries

To overcome the performance issue caused by correlated subqueries, one can convert the subquery into a non-correlated query. This is achieved by selecting all columns in the subquery and then using it as a table in the outer query.

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

By selecting all columns in the subquery, it becomes independent of the values in the parent query and can be executed once to return all duplicate field values.

Modified Query for Improved Performance

Using the non-correlated subquery, the modified query that retrieves all duplicate rows in some_table while avoiding performance issues becomes:

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 approach effectively optimizes the query by eliminating the performance drawbacks associated with correlated subqueries. The query can now retrieve duplicate rows efficiently, allowing the inspection and analysis of potential data anomalies.

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