EXPLAIN select * from t1 LEFT JOIN t2 on t1.c = t2.c 1. 通过执行分析可以看到出现了Using join buffer (Block Nested Loop) Block Nested Loop分析 Block Nested Loop实际上MySQL的一种优化,正常情况下,在没有索引的情况下进行join关联查询,那执行流程应该如下: 1、先从t1查询一行数据。 2、然后到t2中...
engine_condition_pushdown=on,index_condition_pushdown=on,mrr=on,mrr_cost_based=on,block_nested_loop=on,batched_key_access=off,materialization=on,semijoin=on,loosescan=on,firstmatch=on,duplicateweedout=on,subquery_materialization_cost_based=on,use_index_extensions=on,condition_fanout_filter=on...
在EXPLAIN输出中,当Extra值包含Using join buffer(Block Nested Loop)且type值为ALL,index或range时,表示使用BNL。 示例 mysql>explainSELECTa.gender, b.dept_noFROMemployees a, dept_emp bWHEREa.birth_date=b.from_date;+---+---+---+---+---+---+---+---+---+---+---+---+|id|select...
mysql5.7基于块的嵌套循环连接(Block Nested-Loop Join) select*fromt_order t1,t_order_detail t2wheret1.id=t2.order_idandt1.amount>100; 这里t1是驱动表,t2是被驱动表,当t1一行记录对应t2的3行记录时,t2表就需要循环3次,当t1表有N行记录时,t2表就需要循环 N*3次,这种性能是比较低的。 所以mysql就...
| -> Nested loop inner join (cost=2.40 rows=2) -> Filter: (t1.c1 < 3) (cost=1.70 rows=2) -> Index scan on t1 using idx2 (cost=1.70 rows=7) -> Index lookup on t3 using idx3_1 (ccc1=t1.c1) (cost=0.30 rows=1)
Therefore, we instead look to the block-nested loop join (BNLJ); it is well-known that the two algorithms are better than BNLJ, but BNLJ creates no intermediate result writes. It is reasonable to use BNLJ for a major join algorithm if its performance can be enhanced similar to those ...
The block_nested_loop and batched_key_access flags of the optimizer_switch system variable control how the optimizer uses the Block Nested-Loop and Batched Key Access join algorithms. By default, block_nested_loop is on and batched_key_access is off. See Section 8.9.2, “Switchable Optimiza...
使用BKA时,join_buffer_size的值定义了对存储引擎的每个请求中的密钥批次。 缓冲区越大,对连接操作的右侧表进行的顺序访问越多,这可以显着提高性能。 mysql> SET optimizer_switch='mrr=on,mrr_cost_based=off,batched_key_access=on'; 1. There are two scenarios by which MRR functions execute: ...
(cost=1 rows=1.41) (actual time=0.0131..0.0132 rows=1 loops=1) -> Nested loop inner join (cost=0.8 rows=2) (actual time=0.00888..0.0114 rows=2 loops=1) -> Covering index scan on t0 using c10 (cost=0.35 rows=1) (actual time=0.00595..0.00658 rows=1 loops=1) -> Table scan on...
EXPLAIN出力で、Extra値にUsing join buffer (Block Nested Loop)が含まれ、type値がALL、index、またはrangeの場合に、テーブルへの BNL の使用が示されます。 準結合戦略の詳細は、セクション8.2.2.1「準結合変換による IN および EXISTS サブクエリー述語の最適化」を参照してください ...