SELECT 
  `C`.`amo` 
FROM 
  `cscart_categories` AS `C` 
  LEFT JOIN `cscart_products_categories` AS `PC` ON `PC`.`category_id` = `C`.`category_id` 
WHERE 
  `PC`.`product_id` = 30259

Query time 0.00039

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "1.09"
    },
    "nested_loop": [
      {
        "table": {
          "table_name": "PC",
          "access_type": "ref",
          "possible_keys": [
            "PRIMARY",
            "pt",
            "idx_pc_product_id_type"
          ],
          "key": "pt",
          "used_key_parts": [
            "product_id"
          ],
          "key_length": "3",
          "ref": [
            "const"
          ],
          "rows_examined_per_scan": 1,
          "rows_produced_per_join": 1,
          "filtered": "100.00",
          "using_index": true,
          "cost_info": {
            "read_cost": "0.28",
            "eval_cost": "0.10",
            "prefix_cost": "0.38",
            "data_read_per_join": "16"
          },
          "used_columns": [
            "product_id",
            "category_id"
          ],
          "attached_condition": "(`dev2`.`PC`.`category_id` is not null)"
        }
      },
      {
        "table": {
          "table_name": "C",
          "access_type": "eq_ref",
          "possible_keys": [
            "PRIMARY",
            "p_category_id"
          ],
          "key": "PRIMARY",
          "used_key_parts": [
            "category_id"
          ],
          "key_length": "3",
          "ref": [
            "dev2.PC.category_id"
          ],
          "rows_examined_per_scan": 1,
          "rows_produced_per_join": 1,
          "filtered": "100.00",
          "cost_info": {
            "read_cost": "0.62",
            "eval_cost": "0.10",
            "prefix_cost": "1.10",
            "data_read_per_join": "10K"
          },
          "used_columns": [
            "category_id",
            "amo"
          ]
        }
      }
    ]
  }
}