Выбрать в 10 раз медленнее при изменении порядка

Почему этот выбор, назовите его A , (0,02406 с):

select * from char_kills
where rid_first <= 110 and rid_last >= 110
order by kills desc
limit 500;

будет в 10 раз медленнее, чем при обратном порядке сортировки, вызовите it B , (0.00229s):

select * from char_kills
where rid_first <= 110 and rid_last >= 110
order by kills
limit 500;

и как можно оптимизировать A? Использование таблиц InnoDB в MySQL 5.5.

Дополнительная информация приводится ниже.

описать char_kills;

cid, int(10) unsigned, NO, PRI, , 
rid_first, int(10) unsigned, NO, PRI, , 
rid_last, int(10) unsigned, NO, PRI, , 
kills, int(11), NO, PRI, , 
offi_rank, smallint(5) unsigned, YES, , , 

выбрать count (*) из char_kills;

146312

выбрать count (*) из char_kills, где rid_first <= 110 и rid_last> = 110;

7207

показать индексы из char_kills;

char_kills, 0, PRIMARY, 1, kills, A, 160711, , , , BTREE, , 
char_kills, 0, PRIMARY, 2, rid_first, A, 160711, , , , BTREE, , 
char_kills, 0, PRIMARY, 3, rid_last, A, 160711, , , , BTREE, , 
char_kills, 0, PRIMARY, 4, cid, A, 160711, , , , BTREE, , 

A :

Explain:
1, SIMPLE, char_kills, index, , PRIMARY, 16, , 500, Using where

Profiling:
0.02406750, select * from char_kills where rid_first <= 110 and rid_last >= 110 order by kills desc limit 500

starting, 0.000050
checking permissions, 0.000007
Opening tables, 0.000029
System lock, 0.000008
init, 0.000022
optimizing, 0.000008
statistics, 0.000013
preparing, 0.000012
executing, 0.000003
Sorting result, 0.000006
Sending data, 0.023822
end, 0.000007
query end, 0.000004
closing tables, 0.000015
freeing items, 0.000058
logging slow query, 0.000002
cleaning up, 0.000004

B :

Explain:
1, SIMPLE, char_kills, index, , PRIMARY, 16, , 500, Using where

Profiling:
0.00229975, select * from char_kills where rid_first <= 110 and rid_last >= 110 order by kills limit 500

starting, 0.000049
checking permissions, 0.000007
Opening tables, 0.000027
System lock, 0.000008
init, 0.000019
optimizing, 0.000008
statistics, 0.000012
preparing, 0.000009
executing, 0.000003
Sorting result, 0.000004
Sending data, 0.002091
end, 0.000004
query end, 0.000003
closing tables, 0.000010
freeing items, 0.000042
logging slow query, 0.000002
cleaning up, 0.000003

Больше перестановок

Медленно:

where rid_first <= 110 and rid_last >= 110 order by kills desc limit 500; -- 0.031s, A, 7k matching rows
where rid_first >= 110 and rid_last <= 110 order by kills      limit 500; -- 0.094s, 449 rows

Быстро:

where rid_first <= 110 and rid_last >= 110 order by kills      limit 500; -- 0.000s, B
where rid_first <= 110 and rid_last <= 110 order by kills      limit 500; -- 0.000s
where rid_first >= 110 and rid_last >= 110 order by kills desc limit 500; -- 0.000s
where rid_first <= 110 and rid_last <= 110 order by kills desc limit 500; -- 0.000s
where rid_first <= 110 and rid_last <= 110 order by kills desc limit 500; -- 0.000s
where rid_first <= 110                     order by kills desc limit 500; -- 0.000s
where                      rid_last >= 110 order by kills desc limit 500; -- 0.000s