Why the example query with “or” do not use index?

Question

I am reading mysql manual ,

Here is a example in the manual .

The example create a Multiple-Column Indexes ,the index is (last_name,first_name)

CREATE TABLE test (
    id         INT NOT NULL,
    last_name  CHAR(30) NOT NULL,
    first_name CHAR(30) NOT NULL,
    PRIMARY KEY (id),
    INDEX name (last_name,first_name)
);

the manual said that this query will use index

SELECT * FROM test
  WHERE last_name='Widenius' AND first_name='Michael';

but the query with or below will not use index :

SELECT * FROM test
  WHERE last_name='Widenius' OR first_name='Michael';

Question

why the example query with or , namely ,why

SELECT * FROM test
      WHERE last_name='Widenius' OR first_name='Michael';

do not use index ?

Answer 1

Suffice it to say that databases are not good at optimizing OR (or IN or NOT IN ) conditions in the WHERE clause.

At a high level, I might describe the reason as the following. When conditions are connecting using AND , the first narrows the population used for the second. This makes indexes feasible because the conditions "nest". When using OR , the conditions are independent. I should note that some databases can handle OR conditions better than others.

If you want your code to use indexes, you can use UNION ALL :

SELECT t.*
FROM test t
WHERE last_name = 'Widenius'
UNION ALL
SELECT t.*
FROM test t
WHERE last_name <> 'Widenius' AND first_name = 'Michael';

For best performance, you want indexes on test(last_name) and test(first_name, last_name) .