SELECT 
  product_id 
FROM 
  cscart_products_categories 
WHERE 
  product_id = 5640 
  AND category_id = 444

Query time 0.00033

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "message": "Impossible WHERE noticed after reading const tables"
    }
  }
}