TTL of record

[Pavel Francirek]

Hello,

is there any way to update record only once _before_ it expires? Or what
about of idea of getting time-to-live of record? 
We cache some database queries that takes seconds to get. So if memcache
expires it takes hundreds of requests before memcache is filled again.
We use scripts invoked by cron to fill it in shorter period than expire
time. But it means same waste of resources (we do quetries long time
before data expires (to be sure we even use interval several times
shorter than expire time).

Regards,
Pavel