SELECT 
  subscription_id 
FROM 
  cscart_product_subscriptions 
WHERE 
  product_id = 13502 
  AND user_id = 0

Query time 0.00023

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "0.35"
    },
    "table": {
      "table_name": "cscart_product_subscriptions",
      "access_type": "ref",
      "possible_keys": [
        "pe",
        "pd"
      ],
      "key": "pe",
      "used_key_parts": [
        "product_id"
      ],
      "key_length": "3",
      "ref": [
        "const"
      ],
      "rows_examined_per_scan": 1,
      "rows_produced_per_join": 0,
      "filtered": "10.00",
      "cost_info": {
        "read_cost": "0.25",
        "eval_cost": "0.01",
        "prefix_cost": "0.35",
        "data_read_per_join": "79"
      },
      "used_columns": [
        "subscription_id",
        "product_id",
        "user_id"
      ],
      "attached_condition": "(`cscart_db`.`cscart_product_subscriptions`.`user_id` = 0)"
    }
  }
}