MySQL——索引问题

Question

我的情况有点奇怪，我正在尝试理解它。

我有两个表，我将列出必要的字段。

products [MyIsam -- due to full text search]  
   product_id [primary index]

其中大约有 1,000,000 行。

website_products [InnoDB]  
    website_id [primary]  
    product_id [primary]

其中大约有 6,000,000 行。

<小时/>

现在当我进行查询时...

SELECT
    COUNT(`product_id`)
FROM `products`

0.109 秒后返回结果“1030650”

SELECT
    COUNT(wp.`product_id`)
FROM `website_products` AS wp 
WHERE wp.`website_id` = 1133

0.577 秒后返回结果“104150”

SELECT
    COUNT(wp.`product_id`)
FROM `website_products` AS wp 
LEFT JOIN `products` AS p ON p.product_id = wp.product_id
WHERE wp.`website_id` = 1133

38.173 秒后返回结果“104150”

EXPLAIN SELECT 的结果是:

1   SIMPLE  wp  ref PRIMARY,website_id  PRIMARY 4   const   204392  Using index
1   SIMPLE  p   eq_ref  PRIMARY PRIMARY 4   imaginer_system.wp.product_id   1   Using index

为什么我加入 table 后时间变长了？我怎样才能减少这个？

Answer 1

对于此查询:

SELECT COUNT(wp.`product_id`)
FROM `website_products` wp LEFT JOIN
     `products` p
      ON p.product_id = wp.product_id
WHERE wp.`website_id` = 1133;

最佳索引为:website_products(website_id, Product_id) 和 products(product_id)。