If DISTINCT_CALLBACK is non-null, then it will be called exactly
once for every case containing a distinct value of V. AUX is
If DISTINCT_CALLBACK is non-null, then it will be called exactly
once for every case containing a distinct value of V. AUX is
After this function is called, SUBREADER must not ever again
be referenced directly. It will be destroyed automatically
After this function is called, SUBREADER must not ever again
be referenced directly. It will be destroyed automatically