SELECT 
  company_id 
FROM 
  cscart_storefronts_companies 
WHERE 
  storefront_id = 1

Query time 0.00005

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_storefronts_companies",
          "access_type": "ref",
          "possible_keys": ["PRIMARY", "idx_storefront_id"],
          "key": "PRIMARY",
          "key_length": "4",
          "used_key_parts": ["storefront_id"],
          "ref": ["const"],
          "rows": 7,
          "filtered": 100,
          "using_index": true
        }
      }
    ]
  }
}