SELECT 
  1 
FROM 
  cscart_lock_keys 
WHERE 
  key_id = '4f338e80aa7fb4248950f8b626ee53be73d3a1a01517650ea5baad2ddf2731a6' 
  AND expiry_at > UNIX_TIMESTAMP(NOW())

Query time 0.00018

JSON explain

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