SELECT 
  id 
FROM 
  kl_user_follows 
WHERE 
  user_id = 21551 
  AND user_follow_id = 21551

Query time 0.00067

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "158.35"
    },
    "table": {
      "table_name": "kl_user_follows",
      "access_type": "ALL",
      "rows_examined_per_scan": 1571,
      "rows_produced_per_join": 15,
      "filtered": "1.00",
      "cost_info": {
        "read_cost": "156.78",
        "eval_cost": "1.57",
        "prefix_cost": "158.35",
        "data_read_per_join": "251"
      },
      "used_columns": [
        "id",
        "user_id",
        "user_follow_id"
      ],
      "attached_condition": "((`cscart_db`.`kl_user_follows`.`user_follow_id` = 21551) and (`cscart_db`.`kl_user_follows`.`user_id` = 21551))"
    }
  }
}