SELECT 
  category_id 
FROM 
  cscart_products_categories 
WHERE 
  product_id = 2060 
  AND link_type = 'M' 
ORDER BY 
  position 
LIMIT 
  1

Query time 0.00015

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "0.35"
    },
    "ordering_operation": {
      "using_filesort": true,
      "table": {
        "table_name": "cscart_products_categories",
        "access_type": "ref",
        "possible_keys": [
          "link_type",
          "pt"
        ],
        "key": "pt",
        "used_key_parts": [
          "product_id",
          "link_type"
        ],
        "key_length": "6",
        "ref": [
          "const",
          "const"
        ],
        "rows_examined_per_scan": 1,
        "rows_produced_per_join": 1,
        "filtered": "100.00",
        "cost_info": {
          "read_cost": "0.25",
          "eval_cost": "0.10",
          "prefix_cost": "0.35",
          "data_read_per_join": "16"
        },
        "used_columns": [
          "product_id",
          "category_id",
          "link_type",
          "position"
        ]
      }
    }
  }
}