SELECT 
  1 
FROM 
  cscart_lock_keys 
WHERE 
  key_id = 'ebe8ee33b5ce39a11aa2f3000db74583ee5dd08323890fa7bfd1abce7f7a91bb' 
  AND expiry_at > UNIX_TIMESTAMP(NOW())

Query time 0.00019

JSON explain

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