MYSQL query and LIMIT to one row - mysql

Question

very simple but quite confusing to me:
SELECT `start`, `stop` FROM loadtime
WHERE utilisateur_id = '202931999'
AND `type` = 'stat'
AND `stop` !=0
ORDER BY id DESC
LIMIT 0,1
12 sec
SELECT `start`, `stop` FROM loadtime
WHERE utilisateur_id = '202931999'
AND `type` = 'stat'
AND `stop` !=0
ORDER BY id DESC
LIMIT 0,2 or xxx
0.07 sec
explain says
limit 0, 1
id select_type table type possible_keys
1 SIMPLE loadtime index utilisateur_id_2,utilisateur_id,type
key key_len ref rows Extra
PRIMARY 4 NULL 10089 Using where
limit 0, x
id select_type table type possible_keys
1 SIMPLE loadtime ref utilisateur_id_2,utilisateur_id,type
key key_len ref rows Extra
utilisateur_id 62 const 12103 Using index condition; Using where; Using filesort
so the first query doesn't use the index naturally.
server is on MySQL 5.5.32

Answer 1

The problem is simple: for some reason (which can be many) MySQL server decides that in the limit 0,1 case it is a better idea (i.e. the server expects that it would take less time and computation) to do a full table scan in order to give you the right result, instead of using an index.
Since this is a prediction, it could happen that it may be wrong and using an index would work better (as in the limit 0,x case). Since you noticed the performance difference, you can force the query to use the index you think is the best, as follows (and as suggested here):
SELECT `start`, `stop` FORCE INDEX (condition) FROM loadtime
WHERE utilisateur_id = '202931999'
AND `type` = 'stat'
AND `stop` !=0
ORDER BY id DESC
LIMIT 0,1