Re: Vacuum: allow usage of more than 1GB of work mem

On Wed, Sep 14, 2016 at 5:45 AM, Pavan Deolasee
<pavan.deolasee@gmail.com> wrote:
> Another interesting bit about these small tables is that the largest used
> offset for these tables never went beyond 291 which is the value of
> MaxHeapTuplesPerPage. I don't know if there is something that prevents
> inserting more than  MaxHeapTuplesPerPage offsets per heap page and I don't
> know at this point if this gives us upper limit for bits per page (may be it
> does).

From PageAddItemExtended:

    /* Reject placing items beyond heap boundary, if heap */
    if ((flags & PAI_IS_HEAP) != 0 && offsetNumber > MaxHeapTuplesPerPage)
    {
        elog(WARNING, "can't put more than MaxHeapTuplesPerPage items
in a heap page");
        return InvalidOffsetNumber;
    }

Also see the comment where MaxHeapTuplesPerPage is defined:

 * Note: with HOT, there could theoretically be more line pointers (not actual
 * tuples) than this on a heap page.  However we constrain the number of line
 * pointers to this anyway, to avoid excessive line-pointer bloat and not
 * require increases in the size of work arrays.