多個(gè)left join優(yōu)化

tables:

• employee
• employee_orgn:聯(lián)合主鍵(employee_id,orgn_id)，索引1:employee_id,索引2:orgn_id
• orgn

sql:

explain SELECT DISTINCT
    e.*
FROM
    employee e
        LEFT JOIN
    employee_orgn eo ON eo.employee_id = e.id
        LEFT JOIN
    orgn o ON o.id = eo.orgn_id
WHERE
    e.state != 'deleted'
        AND e.state != 'hidden'
        AND (o.state != 'hidden' OR o.state IS NULL)
ORDER BY e.id DESC

explain:

Q：

1. 這里join聯(lián)查，mysql嵌套循環(huán)查詢了10的8次方的數(shù)量級(jí)嗎？
2. 為什么有臨時(shí)表，和排序?yàn)槭裁词俏募判?/li>
3. 為什么第二行是覆蓋索引
4. 希望講解一下這個(gè)explain結(jié)果，優(yōu)化分析

感激不盡呀～