Question

我正在尝试列出特定图书作者的所有book_sales信息。所以我有一个查询，并没有使用索引来查找记录。

以下是我的表格结构：

-- Table structure for table `books`

CREATE TABLE IF NOT EXISTS `books` (
  `book_id` int(11) NOT NULL auto_increment,
  `author_id` int(11) unsigned NOT NULL,
  `book_type_id` int(11) NOT NULL,
  `book_title` varchar(50) NOT NULL,
  `book_price` smallint(4) NOT NULL,
  `in_stock` char(1) NOT NULL,
  PRIMARY KEY  (`book_id`),
  KEY `book_type_id` (`book_type_id`),
  KEY `author_id` (`author_id`)
) ENGINE=InnoDB  DEFAULT CHARSET=latin1 AUTO_INCREMENT=1 ;

-- Dumping data for table `books`

INSERT INTO `books` (`book_id`, `author_id`, `book_type_id`, `book_title`, `book_price`, `in_stock`) VALUES
(1, 1, 1, 'My Book 1', 10, 'y'),
(2, 2, 1, 'My Book 2', 20, 'n'),
(3, 1, 2, 'My Book 3', 30, 'y'),
(4, 3, 3, 'My Book 4', 40, 'y'),
(5, 4, 2, 'My Book 5', 50, 'n'),
(6, 1, 1, 'My Book 6', 60, 'y'),
(7, 5, 3, 'My Book 7', 70, 'n'),
(8, 6, 2, 'My Book 8', 80, 'n'),
(9, 7, 1, 'My Book 9', 90, 'y'),
(10, 8, 3, 'My Book 10', 100, 'n');

-- Table structure for table `book_sales`

CREATE TABLE IF NOT EXISTS `book_sales` (
  `sale_id` int(11) NOT NULL auto_increment,
  `book_id` int(11) NOT NULL,
  `sale_amount` decimal(8,2) NOT NULL default '0.00',
  `time` datetime NOT NULL default '0000-00-00 00:00:00',
  `price` smallint(8) NOT NULL,
  PRIMARY KEY  (`sale_id`),
  KEY `book_id` (`book_id`),
  KEY `price` (`price`)
) ENGINE=InnoDB  DEFAULT CHARSET=latin1 AUTO_INCREMENT=1 ;

-- Dumping data for table `book_sales`

INSERT INTO `book_sales` (`sale_id`, `book_id`, `sale_amount`, `time`, `price`) VALUES
(1, 1, '10.00', '2010-02-23 10:00:00', 20),
(2, 1, '20.00', '2010-02-24 11:00:00', 20);

我的查询：

SELECT sale_amount, price
FROM book_sales
INNER JOIN books ON book_sales.book_id = books.book_id
WHERE books.author_id =1

对上述内容进行说明，向我展示：

id   select_type    table        type        possible_keys      key      key_len    ref     rows      Extra
1   SIMPLE          books       ref     PRIMARY,author_id   author_id   4         const    3        Using index
1   SIMPLE          book_sales  ALL     book_id             NULL        NULL      NULL     2        Using where

显然，book_sales没有使用键'book_id'，尽管我有它。我该怎么做才能使book_sales表使用索引？

谢谢。

根据建议完成修改（但结果是他们仍然不使用索引）：

//Does not use the index in book_sales table
EXPLAIN SELECT sale_amount, price
FROM books, book_sales
FORCE INDEX ( book_id ) 
WHERE book_sales.book_id = books.book_id
AND books.author_id =1

//Does not use the index in book_sales table
EXPLAIN SELECT sale_amount, price
FROM book_sales, books
WHERE books.author_id = 1
AND book_sales.book_id = books.book_id

如何强制book_sale表只有2行，以使用索引？谢谢。

Answer 1

正如您在EXPLAIN中看到的那样，“book_id”被列为可能的密钥。如果MySQL不使用它，那只是优化器不认为它会加速查询。如果“book_sales”只有2行，并且这些行的100％共享相同的“book_id”，则为真。它被称为基数btw。 How to Avoid Table Scans（MySQL手册）

尝试用更多行填充它，您应该看到MySQL将使用索引进行连接。

编辑：查询

SELECT sale_amount, price
FROM books, book_sales
FORCE INDEX ( book_id ) 
WHERE book_sales.book_id = books.book_id
AND books.author_id =1

...在这种情况下也不起作用，因为优化器仍然认识到读取索引是次优的并且切换表顺序以避免这样做。您可以使用STRAIGHT_JOIN强制执行表格顺序。然而，这有点像黑客攻击，因为它迫使MySQL以不是最好的方式执行查询。

      EXPLAIN
       SELECT sale_amount, price
         FROM books
STRAIGHT_JOIN book_sales FORCE INDEX (book_id) ON book_sales.book_id = books.book_id
        WHERE books.author_id = 1

Answer 2

试试这个

SELECT sale_amount, price
FROM book_sales,books
LEFT JOIN books ON(book_sales.book_id = books.book_id)
WHERE books.author_id =1

MySQL Query不使用表连接中的索引

2 个答案: