A typical case for poor IN subquery performance is when the subquery returns a small number of rows but the outer query returns a large number of rows to be compared to the subquery result. The problem is that,
unique_subquery:在对查询结果进行过滤或使用 IN 操作时,优化器会选择使用此类型的查询,使用了 In 操作符的子查询依赖于外层查询的唯一索引。 index_subquery:使用了 In 操作符但子查询使用的普通索引,而不是唯一索引。 range_check:在使用索引来检查外键参照时使用。 index_merge:使用多个索引。 5.3.2.4 Extra us...
A typical case for poor IN subquery performance is when the subquery returns a small number of rows but the outer query returns a large number of rows to be compared to the subquery result. The problem is that, for a statement that uses an IN subquery, the optimizer rewrites it as a co...
subquery performace 2207 Uday V June 21, 2010 06:17PM Re: subquery performace 948 Rick James June 22, 2010 10:28PM Sorry, you can't reply to this topic. It has been closed.Content reproduced on this site is the property of the respective copyright holders. It is not reviewed in ad...
| 2 | DEPENDENT SUBQUERY | abc_number_phone | eq_ref | phone,number_id | phone | 70 | const,func |1| Using where; Using index | +---+---+---+---+---+---+---+---+---+---+ 2 rows in set (0.00 sec) 从上面的信息可以看出,在执行此查询时会扫描两百多万行,难道是没有...
2 DEPENDENT SUBQUERY abc_number_phone eq_ref phone,number_id phone 70 const,func 1 Using where; Using index +---+---+---+---+---+---+---+---+---+---+ 2 rows in set (0.00 sec) 从上面的信息可以看出,在执行此查询时会扫描两百多万行,难道是没有创建索引吗,看一下 mysql>show...
窗口函数(Window Functions)是SQL标准中的一个高级特性,它允许用户在不改变查询结果集行数的情况下,对每一行执行聚合计算或其他复杂的计算。这些计算是基于当前行与结果集中其他行之间的关系进行的。窗口函数特别适用于需要执行跨多行的计算,同时又想保持原始查询结果集的行数不变的场景。
对子查询优化性差,尤其是in(subquery) index merge:同时使用两个index Using union(PRIMARY,idx_fk_film_id);但有时缓存 排序 合并操作会占用大量cpu和内存,使用可以使用IGNORE INDEX禁用某些index 多cpu也无法并行执行 loose index scan:使用两个列组成的index的右侧列做 range scan:Using index for group-by ...
(内层的SELECT语句与外层的SELECT语句有依赖关系)SUBQUERY:代表当前记录是一个子查询的角色存在的DEPENDENT SUBQUERY:代表当前记录是一个相关子查询的角色存在DERIVED:派生表DEPENDENT DERIVED:派生表依赖于另一个表(下文未详细说明)MATERIALIZED:物化表,把子查询查询的数据当作常量UNCACHEABLE SUBQUERY:无法缓存结果且必须为...
in set, 1 warning (0.01 sec) 4- If we use JOIN instead of subquery, it works well mysql [localhost:8040] {msandbox} (test) > explain SELECT main.* FROM t1 main JOIN (SELECT MAX(id) AS max_id FROM t1 WHERE name = 'node' GROUP BY category) subquery ON main.id = subquery.max...