SELECT 
  subscription_id 
FROM 
  cscart_product_subscriptions 
WHERE 
  product_id = 292 
  AND user_id = 0

Query time 0.00036

JSON explain

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