Strategies to cache from MySQL (or any DB I suppose)

[mike]

Obviously it's quite easy to request a single row from a table based
on a key of "table:PK" - however, what if I do not query the table
based on it's PK?

I'd say the majority of the time, my highest traffic queries are not
using a PK (or a compound PK) but a combination of [indexed] columns.
Perhaps this is where I become "re-educated" in my thinking, and you
guys say "hey stupid, instead of sending one database query to your
database, break it up into two" - I *may* be over-engineering this.

Take for example my forum code.

User requests /forum/posts.php?t=$threadid

I select from my database:
SELECT foo,bar,baz FROM posts WHERE threadid=$threadid

Obviously, when checking to see if the cache exists for that, I
suppose I could use "posts:$threadid" for the key. But then I have to
make sure that any time I may modify a row returned from that query, I
have to dirty the cache for any already-cached information with
threadid of $threadid.

Hopefully I'm making sense. Perhaps someone else has some BKMs on how
to change/break up database operations so that I can do caching on
nearly every type of SELECT I will use. Because I'd like to leverage
memcache for everything possible.

File caching is easy - the key would always be the filename (or some
sort of token based on it) - but the database stuff gets interesting
based on how the data is being retrieved, and also how it's updated.

Thanks in advance,
mike