AI代码解释 mysql>explain format=treeSELECT*FROMt1WHEREt1.aIN(SELECTt2.bFROMt2WHEREid<10);***1.row***->Nested loop innerjoin(cost=4.95rows=9)->Filter:(`<subquery2>`.b is notnull)(cost=2.83..1.80rows=9)->Table scan on<subquery2>(cost=0.29..2.61rows=9)->Materializewithdeduplication...
mysql>EXPLAINSELECT*FROMt1WHEREkey1IN(SELECTkey1FROMt2)ORkey3='a1b6cee57a'; 从输出结果中我们可以看到,t1表在外层查询中,外层查询有一个独立的SELECT关键字,所以第一条记录的id值就是1,t2表在子查询中,子查询有一个独立的SELECT关键字,所以第二条记录的id值就是2。 但是这里大家需要特别注意,查询优化器...
Block Nested-Loop Join Algorithm 优化的算法。 8.8.2 EXPLAIN Output Format explain声明提供了MYsql如何执行一个声明的详细信息。 比如select声明,explain会返回:每个表一行信息。 Mysql使用nested-loop join方法来处理所有的join连接。因此,mysql会从第一个表读一行数据,然后在第二个表找到一个匹配的行数据,在然后...
using join buffer Block Nested Loop,需要进行嵌套循环计算。两个关联表join,关联字段均未建立索引,就会出现这种情况。比如内层和外层的type均为ALL,rows均为4,需要循环进行4*4次计算。常见的优化方案是,在关联字段上添加索引,避免每次嵌套循环计算。 本文参考了一些优秀的博客,感兴趣的可以了解下: Explain执行计划 ...
id": 1, # 整个查询语句只有1个SELECT关键字,该关键字对应的id号为1 "cost_info": { "query_cost": "3197.16" # 整个查询的执行成本预计为3197.16 }, "nested_loop": [ # 几个表之间采用嵌套循环连接算法执行 # 以下是参与嵌套循环连接算法的各个表的信息 { "table": { "table_name": "s1", # ...
"nested_loop": [ { "table": { "table_name": "orders", "access_type": "ALL", "possible_keys": [ "fk_item_id" ], "rows_examined_per_scan": 229432, "rows_produced_per_join": 76469, "filtered": "33.33", "cost_info": { ...
Using join buffer : 使用了连接缓存,Block Nested Loop,连接算法是块嵌套循环连接;Batched Key Access,连接算法是批量索引连接。 Using where : 表示 MySQL 服务器从存储引擎收到查询数据,再进行 “后过滤”(Post-filter)。所谓 “后过滤”,就是先读取整行数据,再检查此行是否符合 where 句的条件,符合就留下...
where 'i' is the loop variable The 'do-while' loop can be implemented (in C) as: inti=5; do { printf("%d",i); i--; }while(i>=0); where 'i' is the loop variable. Answer and Explanation:1 Both for loop and while loop can run multiple statements in successive repetition effic...
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In python, the range() function essentially is used with the for loop, it returns a sequence of numbers that begin and end as per the limits specified within the function. For eg: The code snippet below, runs a for loop ranging from lower limit = 0 to upper limit = 10 (exclusive)....