SELECT 
  * 
FROM 
  cscart_product_prices 
WHERE 
  product_id = 2669 
  AND usergroup_id != 0

Query time 0.00056

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "0.70"
    },
    "table": {
      "table_name": "cscart_product_prices",
      "access_type": "ref",
      "possible_keys": [
        "usergroup",
        "product_id",
        "usergroup_id"
      ],
      "key": "product_id",
      "used_key_parts": [
        "product_id"
      ],
      "key_length": "3",
      "ref": [
        "const"
      ],
      "rows_examined_per_scan": 2,
      "rows_produced_per_join": 0,
      "filtered": "17.97",
      "cost_info": {
        "read_cost": "0.50",
        "eval_cost": "0.04",
        "prefix_cost": "0.70",
        "data_read_per_join": "8"
      },
      "used_columns": [
        "product_id",
        "price",
        "percentage_discount",
        "lower_limit",
        "usergroup_id"
      ],
      "attached_condition": "(`dev_yourmart_co_bw`.`cscart_product_prices`.`usergroup_id` <> 0)"
    }
  }
}