SELECT 
  category_id 
FROM 
  cscart_products_categories 
WHERE 
  product_id = 1 
  AND link_type = 'M'

Query time 0.00027

JSON explain

{
  "query_block": {
    "select_id": 1,
    "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": 1,
      "filtered": 100,
      "index_condition": "(`_testcscart`.`cscart_products_categories`.`link_type` = 'M')"
    }
  }
}