> How does it know what to cluster by? Does it gather statistics about
> query patterns on which it can decide an optimal clustering, or does
> it merely follow a clustering previously set up by the user?
Nothing fancy, InnoDB ALWAYS clusters on the primary key, whatever it is.
So, if you can hack your stuff into having a primary key that clusters
nicely, good for you. If not, well...
So, I used (topic_id, post_id) as the PK, even though it isn't the real
PK (this should be post_id)...