SELECT 
  company_id 
FROM 
  orders 
WHERE 
  user_id = 0

Query time 0.01763

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "921.15"
    },
    "table": {
      "table_name": "orders",
      "access_type": "ref",
      "possible_keys": [
        "user_id"
      ],
      "key": "user_id",
      "used_key_parts": [
        "user_id"
      ],
      "key_length": "3",
      "ref": [
        "const"
      ],
      "rows_examined_per_scan": 6564,
      "rows_produced_per_join": 6564,
      "filtered": "100.00",
      "cost_info": {
        "read_cost": "264.75",
        "eval_cost": "656.40",
        "prefix_cost": "921.15",
        "data_read_per_join": "106M"
      },
      "used_columns": [
        "company_id",
        "user_id"
      ]
    }
  }
}