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

Order of Conditions in MySQL WHERE Clauses and Performance

When crafting complex database queries with numerous conditions, the order of those conditions can potentially impact MySQL performance. A common scenario involves a combination of broad and restrictive conditions. The question arises: does it matter if the restrictive condition appears first or last in the WHERE clause?

The answer is not always straightforward. In certain situations, the order can affect performance due to short-circuiting behavior. Consider the following 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 the first query, the company condition is checked last, potentially causing MySQL to search through a large number of rows before narrowing down the results. In the second query, the company condition is evaluated first, which can significantly reduce the number of rows that need to be processed.

This optimization occurs because MySQL uses short-circuiting logic in evaluating the WHERE clause. If the company condition fails for a given row, MySQL will not proceed to evaluate the remaining conditions for that row.

However, it's important to note that this performance benefit may not always be significant, especially if the restrictive condition is not highly selective (i.e., it does not eliminate a large number of rows).

Therefore, while the order of conditions in a WHERE clause can sometimes affect MySQL performance, it is not a general rule that can be applied in all cases. It is best to conduct performance testing on specific queries to determine whether reordering the conditions provides any noticeable improvements.

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