Question

SELECT * FROM table ORDER BY id DESC LIMIT 0,25

当我检查 EXPLAIN 时，它说它查询了1,000,000

注意：表的总行数为1,000,000

有没有办法优化此查询？我只想获得最近的条目（比如25个最近的行）

Answer 1

我不知道这里有很多需要优化的地方。

最糟糕的情况就是这个（我的桌子比你的桌子略大）：

SELECT * FROM n ORDER BY id DESC LIMIT 1048552,25;
+----+
| id |
+----+
| 24 |
| 23 |
| 22 |
| 21 |
| 20 |
| 19 |
| 18 |
| 17 |
| 16 |
| 15 |
| 14 |
| 13 |
| 12 |
| 11 |
| 10 |
|  9 |
|  8 |
|  7 |
|  6 |
|  5 |
|  4 |
|  3 |
|  2 |
|  1 |
+----+
24 rows in set (0.20 sec)

你可以等0.2秒，对吧？

Answer 2

如果您有id的索引，除了EXPLAIN误导之外没有问题：

mysql> SELECT COUNT(*) FROM canada;
+----------+
| COUNT(*) |
+----------+
|     5484 |
+----------+

mysql> FLUSH STATUS;

mysql> SELECT * FROM canada ORDER BY id DESC LIMIT 5;
+--------+---------+----------------+------------------+-------+------------+---------+----------+--------+------------------+
| id     | country | ascii_city     | city             | state | population | lat     | lng      | stripe | province         |
+--------+---------+----------------+------------------+-------+------------+---------+----------+--------+------------------+
| 297207 | ca      |                | (dup population) |       |       6124 |       0 |        0 |      0 | Nunavut          |
| 297206 | ca      | zehner         | Zehner           | 11    |          0 | 50.5667 |  -104.45 |    970 | Saskatchewan     |
| 297205 | ca      | zeballos       | Zeballos         | 02    |          0 | 49.9833 |  -126.85 |    966 | British Columbia |
| 297204 | ca      | zealandia      | Zealandia        | 11    |          0 | 51.6167 |  -107.75 |    977 | Saskatchewan     |
| 297203 | ca      | yukon crossing | Yukon Crossing   | 12    |          0 | 62.3333 | -136.483 |   1051 | Yukon            |
+--------+---------+----------------+------------------+-------+------------+---------+----------+--------+------------------+
5 rows in set (0.01 sec)

mysql> SHOW SESSION STATUS LIKE 'Handler%';
+----------------------------+-------+
| Variable_name              | Value |
+----------------------------+-------+
| Handler_commit             | 1     |
| Handler_external_lock      | 2     |
| Handler_read_key           | 1     |
| Handler_read_last          | 1     |
| Handler_read_prev          | 4     |  -- 1 less than LIMIT 5
+----------------------------+-------+

如果PRIMARY KEY上没有索引（或id），则需要索引。

MYSQL优化表ORDER BY id DESC

2 个答案: