SELECT 
  company_id 
FROM 
  cscart_storefronts_companies 
WHERE 
  storefront_id = 3

Query time 0.00030

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "0.35"
    },
    "table": {
      "table_name": "cscart_storefronts_companies",
      "access_type": "ref",
      "possible_keys": [
        "PRIMARY",
        "idx_storefront_id"
      ],
      "key": "PRIMARY",
      "used_key_parts": [
        "storefront_id"
      ],
      "key_length": "4",
      "ref": [
        "const"
      ],
      "rows_examined_per_scan": 1,
      "rows_produced_per_join": 1,
      "filtered": "100.00",
      "using_index": true,
      "cost_info": {
        "read_cost": "0.25",
        "eval_cost": "0.10",
        "prefix_cost": "0.35",
        "data_read_per_join": "16"
      },
      "used_columns": [
        "storefront_id",
        "company_id"
      ]
    }
  }
}