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

Does the Sequence of Conditions in a MySQL WHERE Clause Impact Performance?

In MySQL queries with extensive WHERE clauses and numerous conditions, it's crucial to optimize performance. Suppose one specific condition, such as a company identifier, significantly reduces the search space from hundreds of thousands to dozens. Should this condition be positioned first in the clause?

Consider two versions of a query that search for clients matching certain parameters:

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) 

Impact on Performance

Traditionally, it was believed that the order of conditions had minimal to no impact on MySQL performance. However, recent testing and insights have revealed that in certain scenarios, it can indeed make a difference.

Short-Circuiting and Order of Conditions

The key factor is short-circuiting. In MySQL, when evaluating compound conditions connected by AND or OR, the database engine may evaluate the first operand only if the condition is relevant. This optimization can save significant time if the first condition narrows down the search space substantially.

Demonstration

To illustrate this effect, consider the following queries:

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

-- query #2
SELECT myint FROM mytable WHERE myslowfunction('query #2', myint) = 1 OR myint >= 3;

The only difference is the order of operands in the OR condition. myslowfunction is a deliberately expensive function that includes the side effect of logging each invocation.

When these queries are executed:

myslowfunction called for query #1 with value 1
myslowfunction called for query #1 with value 2
myslowfunction called for query #2 with value 1
myslowfunction called for query #2 with value 2
myslowfunction called for query #2 with value 3
myslowfunction called for query #2 with value 4

As seen in the trace, myslowfunction is called fewer times for query #1 because the first condition (myint >= 3) eliminates many rows that do not satisfy it. Placing this condition first allows for early termination and prevents unnecessary evaluation of the slow function.

Conclusion

Therefore, the answer to the question is: Yes, the order of conditions in a WHERE clause can sometimes affect MySQL performance due to short-circuiting. By placing restrictive conditions first, database engines can optimize query execution by avoiding unnecessary processing and maximizing efficiency.

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