SELECT 
  company_id 
FROM 
  cscart_storefronts_companies 
WHERE 
  storefront_id = 1

Query time 0.00013

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_storefronts_companies",
          "access_type": "system",
          "possible_keys": ["PRIMARY", "idx_storefront_id"],
          "rows": 1,
          "filtered": 100
        }
      }
    ]
  }
}