When joining to a very small/empty table why MySQL makes a full scan in spite of I'm using “LIMIT”?

Question

EDIT: I removed the GROUP BY clause from the example queries but the same problem shows \"When I join table x to an empty/1 row table y MySQL makes a full table

Answer 1

After some tests it turns out that if the second table(user_school_mm) has some data MySQL will not make full table scan on the first table, and if the second table(country) has no data/very little data (1 or 2 records) MySQL will do a full table scan. Why this happens? I don't know.

How to reproduce

1- Create a schema like this

CREATE TABLE `event` (
   `ev_id` int(11) NOT NULL AUTO_INCREMENT,
   `ev_note` varchar(255) DEFAULT NULL,
   PRIMARY KEY (`ev_id`)
 ) ENGINE=InnoDB;

CREATE TABLE `table1` (
   `id` int(11) NOT NULL AUTO_INCREMENT,
   `name` varchar(45) DEFAULT NULL,   
   PRIMARY KEY (`id`)
 ) ENGINE=InnoDB ;

CREATE TABLE `table2` (
   `id` int(11) NOT NULL AUTO_INCREMENT,
   `name` varchar(45) DEFAULT NULL,   
   PRIMARY KEY (`id`)
 ) ENGINE=InnoDB ;

2- insert in the main table (event in this case) some data (I filled it with 35601000 rows)

3- leave table1 empty and insert 15 rows in table2

insert into table2 (name) values 
('fooBar'),('fooBar'),('fooBar'),('fooBar'),('fooBar'),
('fooBar'),('fooBar'),('fooBar'),('fooBar'),('fooBar'),
('fooBar'),('fooBar'),('fooBar'),('fooBar'),('fooBar');

4- now join the main table with table2 and retest the same query with table1

Query 1 (Fast)

select * 
from 
    event left join 
    table2 on event.ev_id = table2.id
order by event.ev_id
limit 2;
-- executed in 300 milliseconds measured by the client

Explain

+---+-----------+--------+------+----------------+--------+---------+------------------+------+--------+
|id |select_type|table   | type | possible_keys  | key    | key_len | ref              | rows | Extra  |
+---+-----------+--------+------+----------------+--------+---------+------------------+------+--------+
|1  |SIMPLE     |event   |index |                |PRIMARY |4        |                  | 2    |        |
|1  |SIMPLE     |table2  |eq_ref|PRIMARY         |PRIMARY |4        |tests.event.ev_id | 1    |        |
+---+-----------+--------+------+----------------+--------+---------+------------------+------+--------+

Query 2 (Slow)

select * 
from 
    event left join 
    table1 on event.ev_id = table1.id
order by event.ev_id
limit 2;
-- executed in 79 seconds measured by the client

Explain

+---+-----------+--------+------+----------------+--------+---------+-------+---------+---------------------------------------------------+
|id |select_type|table   | type | possible_keys  | key    | key_len | ref   | rows    | Extra                                             |
+---+-----------+--------+------+----------------+--------+---------+-------+---------+---------------------------------------------------+
|1  |SIMPLE     |event   |ALL   |                |        |         |       |33506704 | Using temporary; Using filesort                   |
|1  |SIMPLE     |table1  |ALL   |PRIMARY         |        |         |       |1        | Using where; Using join buffer (Block Nested Loop)|
+---+-----------+--------+------+----------------+--------+---------+-------+---------+---------------------------------------------------+

MySQL version is 5.6.38