Nested Loop Semi Join节点表示同名的连接方法。该计划(就像上面的反连接)提供了seats表的行数的估计(149),尽管仅检索其中一个就足够了。当然实际的查询会在获取一个后停止。 => EXPLAIN (analyze, costs off, timing off, summary off) SELECT * FROM aircrafts a WHERE EXISTS ( SELECT * FROM seats s W...
Oracle在表格联集(Table Join)技術上大致可以分為Nested Loop Join、Hash Join、Merge Join、Semi Join和Anti Join等5種。其中Semi和Anti Join是被運用在子查詢(subquery),另外三種联集方式則被廣泛運用在一般的表格联集上。 hash join(HJ)是一种用于equi-join(而anti-join就是使用NOT IN时的join)的技术。在Or...
int ObNestedLoopJoinOp::inner_get_next_row() { int ret = OB_SUCCESS; if (OB_UNLIKELY(LEFT_SEMI_JOIN == MY_SPEC.join_type_ || LEFT_ANTI_JOIN == MY_SPEC.join_type_)) { // 处理半连接,具体内容略 } else { state_operation_func_type state_operation = NULL; state_function_func_typ...
If the optimizer changes a subquery to a nested-loop join, it might use a variation of the nested-loop join, called asemi join. In a semi join, the database server reads the inner table only until it finds a match. In other words, for each row in the outer table, the inner table...
If the join condition does not match any index, create a new index to use the nested loop join algorithm. Do not use the nested loop join algorithm for a right join, full join, right semi join, or right anti join.
5.6版本及以后,优化器管理参数optimizer_switch中中的block_nested_loop参数控制着BNL是否被用于优化器。默认条件下是开启,若果设置为off,优化器在选择 join方式的时候会选择NLJ算法。 四 参考资料 5.6 版本BNL 支持outer join and semi-join,并且和其他的特性比如BKA 相关联,后面会写文章整理其他的优化点。
Batched Key Access (BKA) Join算法通过index和join buffer访问joined表,BKA算法支持inner join,outer join 和semi join操作,包括嵌套的outer join,BKA的好处包括提高join性能(由于更有效率的table scan),同时,前文的Block Nested loop(BNL)join 算法被扩展也支持这些join类型。
First Step: First table left join second table Second Step: Second table left semi join first table Third Step: Concatenate the results of both output It is very interesting to note how the Nested Loop Join supports the full outer join. An alternative way to mimic this is by converting th...
Nested-LoopJoin Algorithms 8.2.1.6Nested-LoopJoin AlgorithmsMySQL executes joins between tables using anested-loopalgorithm or variations on it.MySQL使用嵌套循环算法或其变体在表之间执行连接。Nested-LoopJoin Algorithm 嵌套循环连接算法A simplenested-loopjoin (NLJ) algorithm reads rows from the first ...
The original implementation of the MySQL BNL algorithm was extended to support outer join and semijoin operations (and was later superseded by the hash join algorithm; seeSection 10.2.1.4, “Hash Join Optimization”). When these operations are executed with a join buffer, each row put into the...