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.
Howeverwe constrain the number of line* pointers to this anyway, to avoid excessive line-pointer bloat and not* require
increasesin the size of work arrays.
 

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company