mysql 官方文档之Optimization（ 8.2 Optimizing SQL Statements）

索引下推

Block Nested-Loop and Batched Key Access Joins

BNL

 开启bka

SET optimizer_switch=‘mrr=on,mrr_cost_based=off,batched_key_access=on‘;

Multi-Range Read Optimization

condition filter （目的是更准确的选择前导表）

 Engine Condition Pushdown Optimization

Hash Join Optimization（无索引，等值连接）

 Beginning with MySQL 8.0.18, MySQL employs a hash join for any query for which each join has an equi-join condition and uses no indexes