SELECT 
  1 
FROM 
  cscart_lock_keys 
WHERE 
  key_id = '51ca7a3716f31ca7e3174e25313d701ab00042f48922fdb702cdfd6767c27cd4' 
  AND expiry_at > UNIX_TIMESTAMP(NOW())

Query time 0.00049

JSON explain

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