How Does WHERE Clause Condition Ordering Impact MySQL Query Performance?

Where Clause Condition Ordering and MySQL Performance

In scenarios where a query contains numerous conditions and a specific condition significantly narrows down the result set, the order of conditions in the WHERE clause can impact MySQL performance.

Consider the following scenarios:

SELECT * FROM clients WHERE
(firstname LIKE :foo OR lastname LIKE :foo OR phone LIKE :foo) AND
(firstname LIKE :bar OR lastname LIKE :bar OR phone LIKE :bar) AND
company = :ugh;

SELECT * FROM clients WHERE
company = :ugh AND
(firstname LIKE :foo OR lastname LIKE :foo OR phone LIKE :foo) AND
(firstname LIKE :bar OR lastname LIKE :bar OR phone LIKE :bar);

In the first query, the condition that filters based on the company field is placed at the beginning of the WHERE clause. This condition effectively narrows down the number of rows that need to be evaluated for the subsequent conditions.

In the second query, the ordering of conditions is reversed. The condition that filters based on the company field is evaluated after the other conditions.

Depending on the distribution of data, the order of conditions can affect performance. By placing the condition that significantly reduces the result set at the beginning of the WHERE clause, MySQL can apply it early and avoid evaluating subsequent conditions for rows that do not meet that condition.

This optimization is known as "short-circuiting." If the condition in the beginning evaluates to false for a given row, MySQL will immediately discard that row without evaluating the remaining conditions.

However, it's important to note that this optimization is not always applicable. If all the conditions have similar selectivity, the order of conditions may not have a significant impact on performance.

To determine the optimal ordering of conditions, consider the following factors:

• Selectivity of each condition
• Distribution of data
• Query execution plan and timing

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