SELECT 
  company_id 
FROM 
  cscart_storefronts_companies 
WHERE 
  storefront_id = 1

Query time 0.00018

Explain
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE cscart_storefronts_companies ref PRIMARY,idx_storefront_id PRIMARY 4 const 1 Using index

Result

company_id
3
4