"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?
