SELECT 
  product_id 
FROM 
  cscart_multi_decproducts 
WHERE 
  product_type = 'D'

Query time 0.00034

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "84.92"
    },
    "table": {
      "table_name": "cscart_multi_decproducts",
      "access_type": "ALL",
      "rows_examined_per_scan": 313,
      "rows_produced_per_join": 31,
      "filtered": "10.00",
      "cost_info": {
        "read_cost": "78.66",
        "eval_cost": "6.26",
        "prefix_cost": "84.92",
        "data_read_per_join": "137K"
      },
      "used_columns": [
        "product_id",
        "product_type"
      ],
      "attached_condition": "(`vishalecarter_multi_dec`.`cscart_multi_decproducts`.`product_type` = 'D')"
    }
  }
}