MySQL 用索引解释过滤列跳跃 4,100

Question

我的查询:

EXPLAIN EXTENDED SELECT  `artwork`.`id` ,  `artwork`.`added` 
FROM  `artwork` 
ORDER BY  `artwork`.`added` DESC 
LIMIT 0 , 6

当我在“added”上添加索引以避免使用 filesort 并使用 index 而不是 explained 的输出时

id select_type table type possible_keys key key_len ref rows filtered Extra
1 SIMPLE artwork ALL NULL NULL NULL NULL 302 100.00 Using filesort

到

id select_type table type possible_keys key key_len ref rows filtered Extra
1 SIMPLE artwork index NULL added 4 NULL 6 5033.33  

我担心 filtered 会上升大约 4,100 - 我无法在 Google 上找到 filtered 是什么意思

Answer 1

嘿，这实际上是个好消息。它显示您的限制从结果集中删除了多少行。在这种情况下，这将是您的 LIMIT 语句。 See the manual :

The filtered column indicates an estimated percentage of table rows that will be filtered by the table condition. That is, rows shows the estimated number of rows examined and rows × filtered / 100 shows the number of rows that will be joined with previous tables. This column is displayed if you use EXPLAIN EXTENDED.