create table t1 (a int primary key, b int);

insert into t1 values (1,1),(2,2),(3,3);

create table t2 (a int primary key, b int);

insert into t2 values (1,1),(2,2),(3,3),(4,4);

set optimizer_trace=1;

SELECT * FROM t1 LEFT JOIN t2 ON t2.a=t1.a WHERE t2.b < 5;

select * from information_schema.optimizer_trace;

explain extended

SELECT * FROM t1 LEFT JOIN t2 ON t2.a=t1.a WHERE t2.b < 5;

drop table t1,t2;