为什么这个WordPress mysql查询这么快？ （又称解释）

Question

有人可以帮助我理解为什么第二个查询这么快吗？

这来自WordPress数据库，其中wp_posts是具有列ID, post_title, post_status...的表，而wp_postmeta是具有列ID, post_id, meta_key, meta_value的表。 wp_posts.ID对应于wp_postmeta.post_id。

我已经查看了Explain的输出，但是我不清楚。我看到查询以略有不同的顺序执行，但我不真正理解为什么更改顺序或为什么速度如此之快。

我真的很想了解，所以我知道它是否适用于其他查询。

谢谢！

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查询1 ：：〜14秒

SELECT
    COALESCE( SUM( wpm1.meta_value ), 0  ) as package_sum,
    wpm2.meta_value as prp_id
FROM wp_posts
LEFT JOIN wp_postmeta wpm1 ON wpm1.post_id = wp_posts.ID
LEFT JOIN wp_postmeta wpm2 ON wpm2.post_id = wp_posts.ID
LEFT JOIN wp_postmeta wpm3 ON wpm3.post_id = wp_posts.ID
WHERE post_type='voucher'
AND post_status IN ( 'requested', 'disputed', 'issued', 'visit-required', 'claimed', 'witheld', 'approved', 'paid' )
AND wpm1.meta_key='voucher_value'
AND wpm2.meta_key='prp'
AND wpm3.meta_key='supplier'
AND wpm3.meta_value='955'
GROUP BY wpm2.meta_value

EXPLAIN:
-----------------------
id   select_type     table       type        possible_keys               key         key_len     ref                     rows    filtered    Extra
1    SIMPLE          wpm2        ref         post_id,meta_key            meta_key    767         const                   80212   100.00      Using where; Using temporary; Using filesort
1    SIMPLE          wp_posts    eq_ref      PRIMARY,type_status_date    PRIMARY     8           gigabit.wpm2.post_id    1       100.00      Using where
1    SIMPLE          wpm3        ref         post_id,meta_key            post_id     8           gigabit.wpm2.post_id    50      100.00      Using where
1    SIMPLE          wpm1        ref         post_id,meta_key            post_id     8           gigabit.wpm2.post_id    50      100.00      Using where

---

查询2 ：：〜3.0秒

SELECT
    COALESCE( SUM( wpm1.meta_value ), 0  ) as package_sum,
    wpm2.meta_value as prp_id
FROM wp_posts
LEFT JOIN wp_postmeta wpm1 ON wpm1.post_id = wp_posts.ID AND wpm1.meta_key='voucher_value'
LEFT JOIN wp_postmeta wpm2 ON wpm2.post_id = wp_posts.ID AND wpm2.meta_key='prp'
LEFT JOIN wp_postmeta wpm3 ON wpm3.post_id = wp_posts.ID AND wpm3.meta_key='supplier'
WHERE post_type='voucher'
AND post_status IN ( 'requested', 'disputed', 'issued', 'visit-required', 'claimed', 'witheld', 'approved', 'paid' )
AND wpm3.meta_value='955'
GROUP BY wpm2.meta_value

EXPLAIN:
-----------------------
id   select_type     table       type        possible_keys               key         key_len     ref                     rows    filtered    Extra
1    SIMPLE          wpm3        ref         post_id,meta_key            meta_key    767         const                   71970   100.00      Using where; Using temporary; Using filesort
1    SIMPLE          wp_posts    eq_ref      PRIMARY,type_status_date    PRIMARY     8           gigabit.wpm3.post_id    1       100.00      Using where
1    SIMPLE          wpm1        ref         post_id,meta_key            post_id     8           gigabit.wpm3.post_id    50      100.00      Using where
1    SIMPLE          wpm2        ref         post_id,meta_key            post_id     8           gigabit.wpm3.post_id    50      100.00      Using where