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