Mysql 使用大型NOT IN语句优化查询

我试图找到只在某个时间戳之前存在的源站点。这个问题似乎很难胜任这项工作。你知道如何优化或改进索引吗

select distinct sourcesite 
  from contentmeta 
  where timestamp <= '2011-03-15'
  and sourcesite not in (
    select distinct sourcesite 
      from contentmeta 
      where timestamp>'2011-03-15'
  );

sourcesite上有一个索引和时间戳，但查询仍然需要很长时间

mysql> EXPLAIN select distinct sourcesite from contentmeta where timestamp <= '2011-03-15' and sourcesite not in (select distinct sourcesite from contentmeta where timestamp>'2011-03-15');
+----+--------------------+-------------+----------------+---------------+----------+---------+------+--------+-------------------------------------------------+
| id | select_type        | table       | type           | possible_keys | key      | key_len | ref  | rows   | Extra                                           |
+----+--------------------+-------------+----------------+---------------+----------+---------+------+--------+-------------------------------------------------+
|  1 | PRIMARY            | contentmeta | index          | NULL          | sitetime | 14      | NULL | 725697 | Using where; Using index                        |
|  2 | DEPENDENT SUBQUERY | contentmeta | index_subquery | sitetime      | sitetime | 5       | func |     48 | Using index; Using where; Full scan on NULL key |
+----+--------------------+-------------+----------------+---------------+----------+---------+------+--------+-------------------------------------------------+

---

我发现not in并不能很好地优化许多数据库。改为使用左外部联接：

这假设sourcesite从不为null。

这应该可以：

SELECT DISTINCT c1.sourcesite
FROM contentmeta c1
LEFT JOIN contentmeta c2
  ON c2.sourcesite = c1.sourcesite
  AND c2.timestamp > '2011-03-15'
WHERE c1.timestamp <= '2011-03-15'
  AND c2.sourcesite IS NULL

为了获得最佳性能，在ContentMetaSourceSite上有一个多列索引timestamp

---

通常，联接的性能优于子查询，因为派生表不能使用索引。

子查询不需要DISTINCT，也不需要外部查询上的WHERE子句，因为您已经在按not IN进行筛选

尝试：

---

这可以在没有“不在”的情况下完成。因为它是最昂贵的mysql之一operation@MoyedAnsari：NOT IN不是最昂贵的操作。这个问题需要NOT IN或NOT EXISTS子查询或左连接-IS NULL查询。

SELECT DISTINCT c1.sourcesite
FROM contentmeta c1
LEFT JOIN contentmeta c2
  ON c2.sourcesite = c1.sourcesite
  AND c2.timestamp > '2011-03-15'
WHERE c1.timestamp <= '2011-03-15'
  AND c2.sourcesite IS NULL

select distinct sourcesite
from contentmeta
where sourcesite not in (
    select sourcesite
    from contentmeta
    where timestamp > '2011-03-15'
);