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

On Wed, Sep 14, 2016 at 5:45 AM, Pavan Deolasee
<pavan(dot)deolasee(at)gmail(dot)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.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
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