为什么这个 mysql 查询（带有 is null 检查）比另一个查询慢那么多？

Question

I getting in doubt with "IS NULL" MySQL check. I have this 2 queries. The first one 运行s in about 300 seconds. The second one 运行 less then 1 second!

Slow query:

SELECT count(distinct(u.id))
FROM ips_usuario AS u 
JOIN ips_fatura AS f
    ON ((u.id = f.ips_usuario_id) OR
       (u.ips_usuario_id_titular IS NOT NULL AND
        u.ips_usuario_id_titular = f.ips_usuario_id));

Fast query:

SELECT count(distinct(u.id))
FROM ips_usuario AS u 
JOIN ips_fatura AS f
    ON ((u.id = f.ips_usuario_id) OR
       (u.ips_usuario_id_titular = f.ips_usuario_id));

All join conditions use foreign keys indexed columns. The table ips_usuario have about 20.000 records and the table ips_fatura have about 500.000 records.

Answer 1

令我惊讶的是两者都很快。我建议将它们替换为 exists:

SELECT COUNT(*)
FROM ips_usuario u  
WHERE EXISTS (SELECT 1 FROM ips_fatura f WHERE u.id = f.ips_usuario_id) OR
      EXISTS (SELECT 1 FROM ips_fatura f WHERE u.ips_usuario_id_titular = f.ips_usuario_id);

第二个：

SELECT COUNT(*)
FROM ips_usuario u  
WHERE EXISTS (SELECT 1 FROM ips_fatura f WHERE u.id = f.ips_usuario_id) OR
      (u.ips_usuario_id_titular IS NOT NULL AND
       EXISTS (SELECT 1 FROM ips_fatura f WHERE u.ips_usuario_id_titular = f.ips_usuario_id)
      )

对于这两个，您需要两个索引：ips_fatura(ips_usuario_id) 和 ips_fatura(ips_usuario_id_titular)。您可以检查说明以确保 EXISTS 正在使用索引。如果不是，较新版本的 MySQL 使用 IN:

的索引

SELECT COUNT(*)
FROM ips_usuario u  
WHERE u.id IN (SELECT f.ips_usuario_id FROM ips_fatura f) OR
      u.ips_usuario_id_titular IN (SELECT f.ips_usuario_id FROM ips_fatura f);

无论哪种情况（EXISTS 或 IN），目标都是 "semi-join"。也就是说，只对第一行进行匹配而不是所有匹配。这是一个重要的效率，因为它允许查询避免重复删除。

我推测问题出在 or 的优化上——通常这会导致 JOIN 算法效率低下。但是，也许 MySQL 在您的第一种情况下很聪明。但是将 IS NULL 添加到外部 table 会将其关闭。

为什么这个 mysql 查询（带有 is null 检查）比另一个查询慢那么多？

Why this mysql query (with is null check) is so slower than this other one?

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