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Sorting in PostgreSQL (cont)

Sorting comparison operators are obtained via catalog (in Type.o): 

// gets pointer to function via pg_operator
struct Tuplesortstate { ... SortTupleComparator ... };

// returns negative, zero, positive
ApplySortComparator(Datum datum1, bool isnull1,
                    Datum datum2, bool isnull2,
                    SortSupport sort_helper);

Flags in SortSupport indicate: ascending/descending, nulls-first/last.

ApplySortComparator() is PostgreSQL's version of tupCompare()