EXISTS vs. IN in MySQL Subqueries: Which is More Performant?

Subquery Optimization: EXISTS vs IN in MySQL

In MySQL, subqueries using EXISTS and IN can achieve similar results, but with contrasting performance implications. This article delves into the differences between these techniques and explores their impact on query execution times.

Performance Comparison Query

The provided example demonstrates two subqueries that yield identical outcomes using different approaches:

Method 1 (IN)

SELECT
   *       
FROM
   tracker       
WHERE
   reservation_id IN (
      SELECT
         reservation_id                                 
      FROM
         tracker                                 
      GROUP  BY
         reservation_id                                 
      HAVING
         (
            method = 1                                          
            AND type = 0                                          
            AND Count(*) > 1 
         )                                         
         OR (
            method = 1                                              
            AND type = 1                                              
            AND Count(*) > 1 
         )                                         
         OR (
            method = 2                                              
            AND type = 2                                              
            AND Count(*) > 0 
         )                                         
         OR (
            method = 3                                              
            AND type = 0                                              
            AND Count(*) > 0 
         )                                         
         OR (
            method = 3                                              
            AND type = 1                                              
            AND Count(*) > 1 
         )                                         
         OR (
            method = 3                                              
            AND type = 3                                              
            AND Count(*) > 0 
         )
   )

Method 2 (EXISTS)

SELECT
    *                                
FROM
    `tracker` t                                
WHERE
    EXISTS (
      SELECT
        reservation_id                                              
      FROM
        `tracker` t3                                              
      WHERE
        t3.reservation_id = t.reservation_id                                              
      GROUP BY
        reservation_id                                              
      HAVING
        (
            METHOD = 1 
            AND TYPE = 0 
            AND COUNT(*) > 1
        ) 
        OR                                                     
        (
            METHOD = 1 
            AND TYPE = 1 
            AND COUNT(*) > 1
        ) 
        OR                                                    
        (
            METHOD = 2 
            AND TYPE = 2 
            AND COUNT(*) > 0
        ) 
        OR                                                     
        (
            METHOD = 3 
            AND TYPE = 0 
            AND COUNT(*) > 0
        ) 
        OR                                                     
        (
            METHOD = 3 
            AND TYPE = 1 
            AND COUNT(*) > 1
        ) 
        OR                                                     
        (
            METHOD = 3 
            AND TYPE = 3 
            AND COUNT(*) > 0
        )                                             
    )

It is observed that Method 1 (IN) requires approximately 10 seconds for execution, while Method 2 (EXISTS) completes in under a second. This substantial performance disparity warrants exploration.

Subquery Behavior

A closer examination reveals that these subqueries utilize different techniques to identify rows in the tracker table that satisfy specific conditions.

IN Subquery (Method 1): This subquery returns a set of reservation_ids that meet the specified criteria. The outer query then checks if the reservation_id for each row in tracker exists in this set, and returns rows that match.

EXISTS Subquery (Method 2): EXISTS determines whether any rows in the tracker table meet the specified conditions for a given reservation_id. The outer query then evaluates this condition and returns rows for which EXISTS returns true.

Performance Considerations

The performance difference between these approaches stems from their underlying logic and efficiency:

• EXISTS: EXISTS performs a series of group-by operations to determine if any rows meet the condition. This approach is more efficient when the number of rows in the tracker table is large.
• IN: IN compares every reservation_id in the tracker table to the set returned by the subquery, which is computationally more expensive, especially when the subquery returns a large number of rows.

Conclusion

In general, EXISTS is the preferred approach when dealing with large subquery results and it is not sensitive to NULL values in the subquery. IN, on the other hand, can be more efficient when the subquery results are relatively small and NULL values are not a concern.

To further optimize performance, it is recommended to consider the following:

• Use appropriate indexes: Ensure that the reservation_id column in the tracker table has an index for efficient lookups.
• Limit the subquery results: Reduce the number of rows returned by the subquery using techniques like LIMIT or WHERE filtering.

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