SELECT 
  category_id 
FROM 
  cscart_products_categories 
WHERE 
  product_id = 14 
  AND link_type = 'M'

Query time 0.00045

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "27.53"
    },
    "table": {
      "table_name": "cscart_products_categories",
      "access_type": "ALL",
      "rows_examined_per_scan": 268,
      "rows_produced_per_join": 2,
      "filtered": "1.00",
      "cost_info": {
        "read_cost": "27.26",
        "eval_cost": "0.27",
        "prefix_cost": "27.53",
        "data_read_per_join": "42"
      },
      "used_columns": [
        "product_id",
        "category_id",
        "link_type"
      ],
      "attached_condition": "((`mobikul_delivery_boy_livedemo`.`cscart_products_categories`.`product_id` = 14) and (`mobikul_delivery_boy_livedemo`.`cscart_products_categories`.`link_type` = 'M'))"
    }
  }
}