more records takes less time - mysql

Question

This is almost driving me insane
I do the following query:
SELECT * FROM `photo_person` WHERE photo_person.photo_id IN (SELECT photo_id FROM photo_person WHERE `photo_person`.`person_id` ='1')
When I change the id, I get different processing time. Although it's all the same queries and tables.
By changing the person_id I get the following:
-- person_id=1 ( 3 total, Query took 0.4523 sec)
-- person_id=2 ( 99 total, Query took 0.1340 sec)
-- person_id=3 ( 470 total, Query took 0.0194 sec)
-- person_id=4 ( 1,869 total, Query took 0.0024 sec)
I do not understand how with the increase of the number of records/results the query time is lower.
The table structures are very straight forward
UPDATE: I have already disabled mysql query cache, so every time I run the query, I would get the same exact value (of course it varies on the milisecond level but this is can be neglected)
UPDATE: table is MyISAM
CREATE TABLE IF NOT EXISTS `photo_person` (
`entry_id` int(11) NOT NULL AUTO_INCREMENT,
`photo_id` int(11) NOT NULL DEFAULT '0',
`person_id` int(11) NOT NULL DEFAULT '0',
PRIMARY KEY (`entry_id`),
UNIQUE KEY `PhotoID` (`photo_id`,`person_id`),
KEY `photo_id` (`photo_id`),
KEY `person_id` (`person_id`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8 AUTO_INCREMENT=182072 ;
Here is the results of the profiling
+----------+------------+-----------------------------+
| Query_ID | Duration |Query |
+----------+------------+-----------------------------+
| 1 | 0.45541200 | SELECT ...`person_id` ='1') |
| 2 | 0.44833700 | SELECT ...`person_id` ='2') |
| 3 | 0.45587800 | SELECT ...`person_id` ='3') |
| 4 | 0.45074900 | SELECT ...`person_id` ='4') |
+----------+------------+-----------------------------+
now since the number are the same, it must be the caching :(
So the aparently the caching kicks in a certain number of records or bytes
mysql> SHOW VARIABLES LIKE "%cac%";
+------------------------------+------------+
| Variable_name | Value |
+------------------------------+------------+
| binlog_cache_size | 32768 |
| have_query_cache | YES |
| key_cache_age_threshold | 300 |
| key_cache_block_size | 1024 |
| key_cache_division_limit | 100 |
| max_binlog_cache_size | 4294963200 |
| query_cache_limit | 1024 |
| query_cache_min_res_unit | 4096 |
| query_cache_size | 1024 |
| query_cache_type | ON |
| query_cache_wlock_invalidate | OFF |
| table_definition_cache | 256 |
| table_open_cache | 64 |
| thread_cache_size | 8 |
+------------------------------+------------+
14 rows in set (0.00 sec)

Answer 1

How are you testing the query speeds? I suspect it's not an appropriate way. The more you query the table, the more likely MySQL is to do some agressive pre-fetching on the table, meaning further queries on the table will be faster, despite they require scanning more data. The reason it is so is because MySQL will not have to load the pages from disk, since it's already pre-fetched them in memory.
As other people have stated, query cache could also mess up you test's results, especially if they implied re-running the query several times in a row to get an "average" runtime.

Answer 2

Add SQL_NO_CACHE to your query to see if it is the cache that tricks you.
To see what is taking time try to use PROFILING like this:
mysql> SET profiling = 1;
mysql> Your select goes here;
mysql> SHOW PROFILES;

Answer 3

Also, try to use the simpler query:
SELECT * FROM photo_person WHERE `photo_person`.`person_id` ='1'
I don't know if MySQL is optimising or not your query, but logically, your and this are equivalent - except that your uses a subquery - always avoid subqueries where possible