Why is My SQL IN Condition Slower Than '=' When Comparing a Single Value?

SQL IN vs. =: Performance Discrepancies

Overview:

The SQL IN operator checks if a value exists within a specified list. However, performance can suffer significantly when using IN compared to the = operator, especially in certain scenarios.

The Issue:

A specific SQL query demonstrated a substantial performance difference between using IN and =, even when IN was only comparing a single value.

Root Cause:

The performance bottleneck stems from a MySQL optimization flaw. MySQL incorrectly categorizes a subquery within an IN clause as a dependent subquery, instead of an independent one.

Dependent vs. Independent Subqueries:

This misclassification is critical. Dependent subqueries execute repeatedly for each row in the outer query, drastically impacting performance. Independent subqueries, on the other hand, execute only once, and their results are cached for efficiency.

Simplified Analysis:

A simplified version of the original query replicated the performance issue, confirming that the IN clause's subquery was treated as dependent, leading to slower execution.

Resolution:

MySQL's misidentification of the subquery type as dependent is the source of the performance degradation when using IN. This problem is resolved in MySQL 5.6.x and later versions.

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