Optimization analysis of Limit query in MySQL optimization techniques

mysql> SELECT * FROM student LIMIT 1000000,1;
+---------+------------+------------+------------+-------+---------------------+
| id | first_name | last_name | created_at | score | updated_at |
+---------+------------+------------+------------+-------+---------------------+
| 1000001 | kF9DxBgnUi | yLXnPSHJpH | 2019-07-11 | 97 | 2019-07-11 14:29:59 | |
+---------+------------+------------+------------+-------+---------------------+
1 row in set (0.31 sec)

mysql> EXPLAIN SELECT score,first_name FROM student ORDER BY created_at LIMIT 1000000,20 \G
*************************** 1. row ***************************
   id: 1
 select_type: SIMPLE
  table: student
 partitions: NULL
   type: index
possible_keys: NULL
   key: time_source_name
  key_len: 69
   ref: NULL
   rows: 1000001
  filtered: 100.00
  Extra: Using index
1 row in set, 1 warning (0.00 sec)

mysql>

mysql> SELECT score,first_name,last_name FROM student ORDER BY created_at LIMIT 1000000,1;
+-------+------------+------------+
| score | first_name | last_name |
+-------+------------+------------+
| 86 | knKsV2g2fY | WB5qJeLZuk |
+-------+------------+------------+
1 row in set (4.81 sec)

mysql>

mysql> explain SELECT score,first_name,last_name FROM student ORDER BY created_at LIMIT 1000000,1\G
*************************** 1. row ***************************
   id: 1
 select_type: SIMPLE
  table: student
 partitions: NULL
   type: ALL
possible_keys: NULL
   key: NULL
  key_len: NULL
   ref: NULL
   rows: 6489221
  filtered: 100.00
  Extra: Using filesort
1 row in set, 1 warning (0.00 sec)

mysql>

mysql> SELECT student.score,student.first_name FROM student INNER JOIN (SELECT id FROM student ORDER BY created_at LIMIT 1000000,1 ) AS temp USING(id);
+-------+------------+
| score | first_name |
+-------+------------+
| 15 | 2QWZ |
+-------+------------+
1 row in set (0.18 sec)

mysql> EXPLAIN SELECT student.score,student.first_name,last_name FROM student INNER JOIN (SELECT id FROM student ORDER BY created_at LIMIT 1000000,1 ) AS temp USING(id);
+----+-------------+------------+------------+--------+---------------+-----------------+--------+---------+---------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+------------+------------+--------+---------------+-----------------+--------+---------+---------+----------+-------------+
| 1 | PRIMARY | <derived2> | NULL | ALL | NULL | NULL | NULL | NULL | 1000001 | 100.00 | NULL |
| 1 | PRIMARY | student | NULL | eq_ref | PRIMARY | PRIMARY | 4 | temp.id | 1 | 100.00 | NULL |
| 2 | DERIVED | student | NULL | index | NULL | time_source_name | 69 | NULL | 1000001 | 100.00 | Using index |
+----+-------------+------------+------------+--------+---------------+-----------------+--------+---------+---------+----------+-------------+
3 rows in set, 1 warning (0.00 sec)

SELECT score,first_name,last_name,id FROM student WHERE id>=last_id ORDER BY id ASC LIMIT 1