for(inti=5;i>=0;i--) { printf("%d",i); } where 'i' is the loop variable Again, The 'while' loop can be implemented (in C) as: inti=5; while(i>=0) { printf("%d",i); i--; } where 'i' is the loop variable
类似于两表连接中被驱动表的eq_ref访问方法,unique_subquery是针对在一些包含IN子查询的查询语句中,如果查询优化器决定将IN子查询转换为EXISTS子查询,而且子查询可以使用到主键进行等值匹配的话,那么该子查询执行计划的type列的值就是unique_subquery,比如下面的这个查询语句: mysql> EXPLAIN SELECT * FROM s1 WHERE ...
| 1 | SIMPLE | s2 | NULL | ALL | NULL | NULL | NULL | NULL | 9954 | 100.00 | Using join buffer (Block Nested Loop) | +---+---+---+---+---+---+---+---+---+---+---+---+ 2 rows in set, 1 warning (0.01 sec) 可以看到这个连接查询的执行计划中有两条记录,这...
2、执行查询explain select * from a join b on a_id=b_id where b_id in (select c_id from c); mysql> explain select * from a join b on a_id=b_id where b_id in (select c_id from c); EXPLAINSELECT*FROMaJOINbONa_id=b_idWHEREb_idIN(SELECTc_idFROMc); id select_typetablepa...
Using join buffer (Block Nested Loop) 在连接查询执行过程中,当被驱动表不能有效的利用索引加快访问速度,MySQL一般会为其分配一块名叫join buffer的内存块来加快查询速度,也就是我们所讲的基于块的嵌套循环算法,比如下边这个查询语句: mysql> EXPLAIN SELECT * FROM s1 INNER JOIN s2 ON s1.common_field = ...
EXPLAIN SELECT*FROM sc_table WHERE key_1 IN(SELECT key_1 FROM sc_table_1); 如上,查询语句中有一个子查询,但执行计划中两个表的id都是1,说明查询优化器将子查询转换成连接查询。 二、 带UNION子句的查询,每个SELECT关键字同样会对应一个id,但是还有一点不通的特征,如下: ...
一. NESTED LOOP: 对于被连接的数据子集较小的情况,嵌套循环连接是个较好的选择。在嵌套循环中,内表被外表驱动,外表返回的每一行都要在内表中检索找到与它匹配的行,因此整个查询返回的结果集不能太大(大于1 万不适合),要把返回子集较小表的作为外表(CBO 默认外表是驱动表),而且在内表的连接字段上一定要有索...
The join operation can be classified into hash join, nested loop join, and merge join. Hash Join Hash join is a way of joining two or more tables. During a hash join, a hash table is built in the memory based on a to-be-joined table, which is usually a small table. The data ...
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How is it that ADH action functions as a mechanism for controlli Urine formed in the nephrons empties directly into what structure? a. Peritubular capillaries. b. Loop of Henle. c. Collecting duct. d. Urethra. e. Renal pelvis. Why doesn't a...