SELECT 
  1 
FROM 
  cscart_lock_keys 
WHERE 
  key_id = '875dcbe85fb09271a6aa9cc726f28fa770776537d1d34873ab59450e21f19ff6' 
  AND expiry_at > UNIX_TIMESTAMP(NOW())

Query time 0.00017

JSON explain

{
  "query_block": {
    "select_id": 1,
    "message": "no matching row in const table"
  }
}