準(zhǔn)備工作

# 總記錄數(shù)為500000
mysql> select count(id) from edu_test;
+-----------+
| count(id) |
+-----------+
|    500000 |
+-----------+
1 row in set (0.05 sec)

分析過程

從0開始查詢10條：

mysql> select * from edu_test limit 0, 10;

10 rows in set (0.05 sec)

從20萬開始查詢10條：

mysql> select * from edu_test limit 200000, 10;

10 rows in set (0.14 sec)

從50萬開始查詢10條：

mysql> select * from edu_test limit 499000, 10;

10 rows in set (0.21 sec)

• 現(xiàn)象：隨著分頁越深入盈简，查詢的時(shí)間也越來越長救氯。

mysql> explain select * from edu_test limit 200000, 10;
+----+-------------+----------+------------+------+---------------+------+---------+------+--------+----------+-------+
| id | select_type | table    | partitions | type | possible_keys | key  | key_len | ref  | rows   | filtered | Extra |
+----+-------------+----------+------------+------+---------------+------+---------+------+--------+----------+-------+
|  1 | SIMPLE      | edu_test | NULL       | ALL  | NULL          | NULL | NULL    | NULL | 499483 |   100.00 | NULL  |
+----+-------------+----------+------------+------+---------------+------+---------+------+--------+----------+-------+
1 row in set (0.09 sec)

• 思考：limit分頁做了一個(gè)全表掃描驾诈，掃描后將從200000開始往后取10條記錄返回凤瘦。

優(yōu)化

思路：

• 快速定位到要訪問的數(shù)據(jù)行，縮小掃描范圍拂封。
  方案1
• 延遲查詢（先定位再查詢）.
  方案2茬射、方案3

方案1：通過有序唯一索引縮小掃描范圍
前提必須要id有序，要不然結(jié)果會漏掉一部分?jǐn)?shù)據(jù)的烘苹。

mysql> select * from edu_test where id > 499000 order by id asc limit 10;

10 rows in set (0.14 sec)

mysql> explain select * from edu_test where id > 499000 order by id asc limit 10;
+----+-------------+----------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| id | select_type | table    | partitions | type  | possible_keys | key     | key_len | ref  | rows | filtered | Extra       |
+----+-------------+----------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | edu_test | NULL       | range | PRIMARY       | PRIMARY | 4       | NULL | 1000 |   100.00 | Using where |
+----+-------------+----------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
1 row in set (0.16 sec)
# 再縮小掃描范圍
mysql> select * from edu_test where id between 499000 and 499020 order by id asc limit 10;

10 rows in set (0.09 sec)

mysql> explain select * from edu_test where id between 499000 and 499020 order by id asc limit 10;
+----+-------------+----------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| id | select_type | table    | partitions | type  | possible_keys | key     | key_len | ref  | rows | filtered | Extra       |
+----+-------------+----------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | edu_test | NULL       | range | PRIMARY       | PRIMARY | 4       | NULL |   21 |   100.00 | Using where |
+----+-------------+----------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
1 row in set (0.08 sec)

方案2：子查詢

mysql> SELECT * FROM edu_test WHERE id >=  (SELECT id FROM edu_test ORDER BY id LIMIT 499000, 1) LIMIT 10;

10 rows in set (0.16 sec)

mysql> explain SELECT * FROM edu_test WHERE id >=  (SELECT id FROM edu_test ORDER BY id LIMIT 499000, 1) LIMIT 10;
+----+-------------+----------+------------+-------+---------------+---------+---------+------+--------+----------+-------------+
| id | select_type | table    | partitions | type  | possible_keys | key     | key_len | ref  | rows   | filtered | Extra       |
+----+-------------+----------+------------+-------+---------------+---------+---------+------+--------+----------+-------------+
|  1 | PRIMARY     | edu_test | NULL       | range | PRIMARY       | PRIMARY | 4       | NULL |   1000 |   100.00 | Using where |
|  2 | SUBQUERY    | edu_test | NULL       | index | NULL          | PRIMARY | 4       | NULL | 499001 |   100.00 | Using index |
+----+-------------+----------+------------+-------+---------------+---------+---------+------+--------+----------+-------------+
2 rows in set (0.14 sec)

方案3：join查詢

mysql> select * from edu_test s, (select id from edu_test order by id limit 499000, 10) t where s.id = t.id;

10 rows in set (0.16 sec)

mysql> explain select * from edu_test s, (select id from edu_test order by id limit 499000, 10) t where s.id = t.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 | 499010 |   100.00 | NULL        |
|  1 | PRIMARY     | s          | NULL       | eq_ref | PRIMARY       | PRIMARY | 4       | t.id |      1 |   100.00 | NULL        |
|  2 | DERIVED     | edu_test   | NULL       | index  | NULL          | PRIMARY | 4       | NULL | 499010 |   100.00 | Using index |
+----+-------------+------------+------------+--------+---------------+---------+---------+------+--------+----------+-------------+
3 rows in set (0.10 sec)

實(shí)際業(yè)務(wù)場景

• 場景：
  在我們設(shè)計(jì)數(shù)據(jù)庫id的時(shí)候躲株，可能采用字符串格式、有順序的id镣衡，帶有一定的業(yè)務(wù)邏輯這樣的分布式id霜定。
• 解決：
  如果我們分頁想要優(yōu)化時(shí)候，根據(jù)減少掃描思路廊鸥，可以通過where id like '10289%' 方式望浩，先縮小范圍再分頁。
• 啟示：
  在設(shè)計(jì)數(shù)據(jù)庫id主鍵的時(shí)候惰说，盡量保持主鍵唯一且有序磨德，最好能解決熱點(diǎn)業(yè)務(wù)問題（如果依賴很多的非主鍵值，那么我們可能還需要回表操作），而且主鍵本身就是一種唯一索引典挑，這種唯一有序特性可以便于幫助我們后期優(yōu)化酥宴，減少掃描記錄范圍。