Daniel

1. at any transaction level. this is storage engine, innodb has row lock.
2. create table in mysql. create table ex (id integer primary key,text varchar(200));

session 1	session 2
start transaction;
	start transaction;
INSERT INTO ex (id, text) SELECT * FROM (SELECT 2, ‘first’) AS tmp WHERE NOT EXISTS ( SELECT id FROM repexeatable_read WHERE text = ‘first’ ) LIMIT 1;
Query OK, 1 row affected (0.00 sec)
Records: 1 Duplicates: 0 Warnings: 0
	INSERT INTO ex (id, text) SELECT * FROM (SELECT 3, ‘first’) AS tmp WHERE NOT EXISTS ( SELECT id FROM ex WHERE text = ‘first’ ) LIMIT 1;
	result: waiting…
commit;
	Query OK, 0 rows affected (4.94 sec[waiting time])
	commit;
	Query OK, 0 rows affected (0.00 sec)