SELECT 
  category_id 
FROM 
  cscart_products_categories 
WHERE 
  product_id = 19559 
  AND link_type = 'M'

Query time 0.00051

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_products_categories",
      "access_type": "ref",
      "possible_keys": ["link_type", "pt"],
      "key": "pt",
      "key_length": "6",
      "used_key_parts": ["product_id", "link_type"],
      "ref": ["const", "const"],
      "rows": 1,
      "filtered": 100,
      "index_condition": "cscart_products_categories.link_type = 'M'"
    }
  }
}