There is no memcache - there is just TAO, and it talks to the database. TAO is a read/write-through cache, so the only way the data changes in the database is through TAO. TAO contains the indices necessary to answer queries like that efficiently (although there may be other systems for doing similar but more specialized or slightly different queries), as well as a cache of the data.

So, when you add a comment, TAO updates its internal structures with the new comment in the right place (after the DB is updated), and there are no "keys" that need to be updated beyond that.