Question

我有这个查询需要大约40-50秒来加载

SELECT
  members.latitude,
  members.longitude,
  members.mem_id
FROM members
WHERE profile_type != "C"
    AND (gender = 'm'
          OR gender = 'f')
    AND members.mem_id IN(SELECT DISTINCT
                            n2.mem_id
                          FROM network n1,
                            network n2
                          WHERE n1.frd_id = n2.mem_id
                              AND n1.mem_id = '48'
                              AND n2.frd_id = '48')
ORDER BY profilenam

有包含用户朋友的网络表

请帮助..

Answer 1

这很慢因为使用了子查询而没有使用派生表进行子查询：

SELECT latitude, longitude, a.mem_id
FROM members a
     INNER JOIN (SELECT DISTINCT n2.mem_id AS mem_id
                 FROM network n1,
                      network n2
                 WHERE n1.frd_id = n2.mem_id
                       AND n1.mem_id = '48'
                       AND n2.frd_id = '48'
                ) b
                ON a.mem_id = b.mem_id
WHERE profile_type != "C"
      AND (gender = 'm'
           OR gender = 'f')
ORDER BY profilenam;

还要在表上添加适当的索引，并使用EXPLAIN EXTENDED检查索引使用情况：

ALTER TABLE network ADD KEY ix1(mem_id);
ALTER TABLE network ADD KEY ix1(frd_id);

Answer 2

如果结果集很小，IN子句才真正适用。对于更大的结果集，请考虑重写查询。过滤器（IN子句生成的过滤器）是一种昂贵的数据库操作。

Answer 3

用EXISTS替换IN子句。

IN集合中只允许1000项

在（items_num = 1000）

如何优化这个需要花费很多时间的查询？

3 个答案: