伪代码如下(摘自http://blogs.msdn.com/b/craigfr/archive/2006/07/26/nested-loops-join.aspx) for each row R1 in the outer table for each row R2 in the inner table if R1 joins with R2 return (R1, R2) 在最原始下,算法的成本=outer
连接是sql语言的一个关键特性,它们是sql语言灵活性的基础。行的集合(直接从表中检索或者作为其他操作的结果接收)总是成对进行连接。有如下连接类型:Inner join,outer join,anti-join and semi joinInner joins:内连接(由inner join指定或者简化为为join)包括满足特定连接条件的两个集合的行对。连接条件将一组行的...
SQL调优之四:嵌套循环连接(Nested Loops Joins) 嵌套循环连接 嵌套循环连接一个外部数据集到内部数据集中,针对外部数据集的每一行数据,数据库会去匹配内部数据集中所有匹配谓词条件的行。如果这个时候内部数据集或者内部表有索引可用,那么数据库就会通过使用它来定位rowid来获取数据。 优化器什么时候考虑使用嵌套循环连接?
For the query with inner joins, the optimizer could choose a different order of nested loops, such as this one: FOR each row t3 in T3 { FOR each row t2 in T2 such that P2(t2,t3) { FOR each row t1 in T1 such that P1(t1,t2) { IF P(t1,t2,t3) { t:=t1||t2||t3; OUTPU...
For the query with inner joins, the optimizer could choose a different order of nested loops, such as this one: FOR each row t3 in T3 { FOR each row t2 in T2 such that P2(t2,t3) { FOR each row t1 in T1 such that P1(t1,t2) { IF P(t1,t2,t3) { t:=t1||t2||t3; OUTPU...
if R1 joins with R2 return (R1, R2) if R1 did not join return (R1, NULL) end This algorithm keeps track of whether we joined a particular outer row. If after exhausting all inner rows, we find that a particular inner row did not join, we output it as a NULL extended row. ...
inner_loop 优化器模式为FIRST_ROWS时,我们经常会发现有大量的NESTED LOOP 这时,在返回数据给用户时,我们没有必要缓存任何数据,这是nested loop的一大亮点 1. 2. 3. 4. 5. 6. 4.使用场景 一般用在连接的表中有索引,并且索引选择性较好(也就是Selectivity接近1)的时候 ...
我们都知道SQL的join关联表的使用方式,但是这次聊的是实现join的算法,join有三种算法,分别是Nested Loop Join,Hash join,Sort Merge Join。 MySQL官方文档中提到,MySQL只支持Nested Loop Join这一种join algorithm MySQL resolves all joins using a nested-loop join method. This means that MySQL reads a row fr...
for each row R2 in the inner table if R1 joins with R2 return (R1, R2) if R1 did not join return (R1, NULL) end" The part in Red is not correct. If this is a Left Join and R2 is the inner table, then If R1 did not join, it should return (R2, NULL).. Is'nt it??
2011-11-21 16:13 −1.嵌套循环连接(Nested Loops)适用范围两个表, 一个叫外部表, 一个叫内部表. 如果外部输入非常小,而内部输入非常大并且已预先建立索引,那么嵌套循环联接将特别有效率。 关于连接时哪个表为outer表,哪个为inner表,我发现sql server会自动给你安排,和你写的位置无关,它自动... ...