Re: MaxOffsetNumber versus MaxHeapTuplesPerPage

Heikki Linnakangas <heikki(at)enterprisedb(dot)com> writes:
> Zdenek Kotala wrote:
>> Is there any reason to have both these macros? By my opinion
>> MaxHeapTuplesPerPage is more accurate and it should replace all
>> MaxOffsetNumber occurrence.

> We use MaxOffsetNumber with index pages as well.

> At quick glance, the only places I can see where we could replace
> MaxOffsetNumber with MaxHeapTuplesPerPage, are in vacuum.c and
> vacuumlazy.c, where we allocate arrays big enough to hold potentially a
> full page's worth of tuples. We could change those, but it's hardly
> worth the trouble.

There is also a difference in intent: MaxOffsetNumber is selected so
that it's physically impossible to have more than that many offsets on a
page, and so it's safe to use an array sized that way without any
overflow checks. MaxHeapTuplesPerPage is the most that *should* be
there but one can think of corner cases where there could be more (eg,
limit on number of redirect pointers hasn't been enforced correctly,
not to mention flat-out corrupt page). If there is any code using
MaxHeapTuplesPerPage as an array size and not backstopping it with an
explicit overflow check, that would be a bug IMHO.

regards, tom lane