To enable it, set to true when creating a new iterator. Note that tailing iterator currently supports only moving in the forward direction (in other words, Prev() and are not supported).

A tailing iterator provides a merged view of two internal iterators:

Since SST files and immutable memtables don’t change, a tailing iterator can often get away by performing a seek operation only on the mutable iterator. For this purpose, it maintains the interval (prev_key, current_key] currently covered by the immutable iterator (in other words, there are no records with key such that prev_key < k < current_key neither in SST files nor immutable memtables). Therefore, when Seek(target) is called and is within that interval, immutable iterator is already at the correct position and it is not necessary to move it.