I assume the indexes are laid out like (col, timestamp)? Can you handle an access pattern where keys are continuously inserted with the same value, and then deleted shortly after? Will queries for the value work efficiently, or have to scan all rows inserted with that value for the last 24 hours? Is there a way to tweak GC to run more aggressively on a particular table (knowing that you wouldn't be able to query back in time)?
We have an access pattern like that in our database. Rows are inserted into the table with x=null, then later x is set to some unique value, but we need to query where x is null in order to set the actual value. At one point we had a long running transaction that prevented the database from cleaning up old records, so the query for X is null grew slower and slower to the point where it almost browned us out.