How to understand how sql query works?

Question

MySQL

SELECT `group_id` FROM `vkgroups` WHERE `group_id` 
IN (SELECT * FROM (SELECT `group_id` FROM `vkmembers` ORDER BY `members` DESC LIMIT 0,20) temp_tab)

The first request is executed for 3 seconds, if it is repeated then 0.0001 Then we change LIMIT for example to 20.20 or 40.20 everything repeats - the first request is 2-3 seconds, the rest are very fast.
EXPLAIN:

Profiling results:

Answer 1

Requests are cached

mysql> show variables like 'query_cache%';
+------------------------------+----------+
| Variable_name                | Value    |
+------------------------------+----------+
| query_cache_limit            | 10485760 |
| query_cache_min_res_unit     | 4096     |
| query_cache_size             | 52428800 |
| query_cache_type             | ON       |
| query_cache_wlock_invalidate | OFF      |
+------------------------------+----------+
5 rows in set (0.20 sec)