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How Does WHERE Clause Condition Ordering Affect MySQL Query Performance?
How Does WHERE Clause Condition Ordering Affect MySQL Query Performance?
Dec 29, 2024 pm 12:31 PM
The Impact of WHERE Clause Condition Ordering on MySQL Performance
When crafting complex queries with multiple conditions, the order of those conditions can potentially influence MySQL's performance. A particular condition, such as filtering by company ID, can significantly reduce the number of rows examined.
Question:
Consider the following two queries with differing condition ordering:
Query 1:
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
Query 2:
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)
Does the order of these conditions impact MySQL's efficiency?
Answer:
The order of WHERE clause conditions can indeed affect MySQL performance under certain circumstances. Consider the following example:
Query 3:
SELECT myint FROM mytable WHERE myint >= 3 OR myslowfunction('query #1', myint) = 1;
Query 4:
SELECT myint FROM mytable WHERE myslowfunction('query #2', myint) = 1 OR myint >= 3;
The only difference between these queries is the order of operands in the OR condition. "myslowfunction" is a function that intentionally delays for a second. The query execution reveals that:
- Query 3: myslowfunction is called four times (myint values 1, 2, 3, 4).
- Query 4: myslowfunction is called seven times (myint values 1, 2, 3, 4, 5, 6, 7).
This demonstrates that when a slow function appears on the left side of an OR condition with an operand that isn't always true, the function is executed more often.
Conclusion:
In summary, the order of conditions in a WHERE clause can influence MySQL performance due to short-circuiting. To optimize performance, place conditions that are expected to significantly reduce the number of rows examined (e.g., company ID) as early as possible in the condition chain.
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