"For BAT-sets that contain exactly the same sets of OIDs, those OID-columns can be marked synced(). This information is then used to use positional lookup (the i-th element in BAT1 corresponds with the i-th in BAT2) and to turn semijoin- into very cheap copy-operations, etc." almost makes it sound as if you need synched() to actual test and set this bit. Is this true? How is the "syncedness" ensured when I have a BAT[OID, any], or determined?