SELECT 
  `product_id` 
FROM 
  `cscart_guest__compare` 
WHERE 
  `guest_id` = 0

Query time 0.00041

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "9.05"
    },
    "table": {
      "table_name": "cscart_guest__compare",
      "access_type": "ALL",
      "rows_examined_per_scan": 88,
      "rows_produced_per_join": 8,
      "filtered": "10.00",
      "cost_info": {
        "read_cost": "8.17",
        "eval_cost": "0.88",
        "prefix_cost": "9.05",
        "data_read_per_join": "140"
      },
      "used_columns": [
        "guest_id",
        "product_id"
      ],
      "attached_condition": "(`dev2`.`cscart_guest__compare`.`guest_id` = 0)"
    }
  }
}