MYSQL Optimize table ORDER BY id DESC

Question

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

when I checked the EXPLAIN it says that it queried 1,000,000

note: total rows of table is 1,000,000

Is there a way to optimize this query? I only wanted to get the most recent entry (let say 25 recent rows)

Answer 1

I don't see that there's much here that needs optimizing.

The worst case scenario is this (I have a very slightly larger table than yours):

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)

You can wait 0.2 secs, right?

Answer 2

If you have an index on id , there is not a problem, other than EXPLAIN being misleading:

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
+----------------------------+-------+

If you don't have an index (or PRIMARY KEY ) on id , you need one.