mysql - Why the difference in performance when using one index versus multiple indexes -

let's have table named impression having 3 fields

id  site_id  timestamp

all 3 fields int. have run following query

select count( * ) c impression  timestamp<unix_timestamp(str_to_date('09,07,2009','%d,%m,%y'))  , site_id=11

findings

if define 2 separate indexes, 1 on timestamp , 1 on site id results slower. on data set result takes 0.13 s calculate.

however if define 1 composite index includes both fields in 1 results faster 0.0002 s

question

why indexed fields have under 1 index? if have 2 separate indexes them why don't both of them used

note

yes explain query that's not question, explain suggests observed, why have 1 index per query

Bready