How Does the Order of WHERE Clause Conditions Affect MySQL Query Performance?

Impact of WHERE Clause Condition Order on MySQL Performance

When crafting complex queries with numerous conditions, the order in which you specify those conditions can impact MySQL performance. While the order generally makes little difference for simple conditions, it becomes more significant when one condition significantly narrows down the result set.

Consider the following two queries:

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 both queries, we search for rows meeting specific criteria across multiple fields using OR conditions. While the first condition typically yields a large result set, the "company" condition restricts the results to a much narrower set.

According to MySQL's execution plan, it follows a left-to-right evaluation order for WHERE conditions. This means that in the first query, MySQL will execute the "company" condition before applying the OR conditions. If the "company" condition returns a small number of rows, MySQL will index them for efficiency, reducing the overhead of processing the subsequent OR conditions.

In contrast, the second query has the "company" condition listed after the OR conditions. As a result, MySQL will execute the OR conditions first, potentially processing a larger number of rows and creating a temporary index if needed. This additional indexing step can increase query overhead.

Short-Circuiting and Performance Impact

It's important to note that the order of WHERE clause conditions can also affect performance in cases where short-circuiting occurs. When a condition is evaluated to be true, MySQL immediately returns true for the entire condition, bypassing the evaluation of subsequent conditions.

For example, consider the following query:

SELECT myint FROM mytable WHERE myint >= 3 OR myslowfunction('query #1', myint) = 1;

If "myint >= 3" evaluates to true for a given row, MySQL will not execute the "myslowfunction" call, which is a time-consuming operation. However, if "myslowfunction" appears on the left side of the OR condition, it will be executed for all rows, even those that do not meet the "myint >= 3" condition.

Conclusion

While the order of WHERE clause conditions generally has minimal performance impact, it can become a factor in cases where one condition significantly narrows down the result set or where short-circuiting is involved. By placing the more restrictive conditions toward the beginning of the WHERE clause, you can potentially improve query performance and efficiency.

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