SELECT 
  * 
FROM 
  cscart_products_categories 
WHERE 
  product_id = 420

Query time 0.00030

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "25.20"
    },
    "table": {
      "table_name": "cscart_products_categories",
      "access_type": "ref",
      "possible_keys": [
        "pt"
      ],
      "key": "pt",
      "used_key_parts": [
        "product_id"
      ],
      "key_length": "3",
      "ref": [
        "const"
      ],
      "rows_examined_per_scan": 21,
      "rows_produced_per_join": 21,
      "filtered": "100.00",
      "cost_info": {
        "read_cost": "21.00",
        "eval_cost": "4.20",
        "prefix_cost": "25.20",
        "data_read_per_join": "336"
      },
      "used_columns": [
        "product_id",
        "category_id",
        "link_type",
        "position",
        "category_position"
      ]
    }
  }
}