Rails控制台vs MySQL控制台-计算巨大表中的行

Question

I'd like to show a graph on how many records a table has, for the last 30 days. I'm going to do a rake task that I will run each day from a cron job.

I just made a test, and I found a weird "issue". I ran this code in the Rails console:

SeenEpisode.count
#=> (57135.2ms)  SELECT COUNT(*) FROM `seen_episodes`

I then tested this in the MySQL console:

use my_database;
SELECT COUNT(*) FROM seen_episodes;
1 row in set (3.94 sec)

This table has 21 million records.

I ran both tests on my production server, to get real performance numbers.

Other tables with around 500k records, take 80-300ms in the Rails console.

Why does counting (using the same query) the 21m big table in Rails take so much longer than in the MySQL console?

Answer 1

我猜您是在INNODB上，其中不缓存count（*）。