SELECT 
  1 
FROM 
  cscart_lock_keys 
WHERE 
  key_id = '21ac80a4531fdb10c541880204d6edfec23d8b9f764858e2f1b9e9cbc75c17b1' 
  AND expiry_at > UNIX_TIMESTAMP(NOW())

Query time 0.00022

JSON explain

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