Re: PATCH: Using BRIN indexes for sorted output

On 2/24/23 09:39, Alvaro Herrera wrote:
> On 2023-Feb-23, Matthias van de Meent wrote:
> 
>>> + for (heapBlk = 0; heapBlk < nblocks; heapBlk += pagesPerRange)
>>
>> I am not familiar with the frequency of max-sized relations, but this
>> would go into an infinite loop for pagesPerRange values >1 for
>> max-sized relations due to BlockNumber wraparound. I think there
>> should be some additional overflow checks here.
> 
> They are definitely not very common -- BlockNumber wraps around at 32 TB
> IIUC.  But yeah, I guess it is a possibility, and perhaps we should find
> a way to write these loops in a more robust manner.
> 

I guess the easiest fix would be to do the arithmetic in 64 bits. That'd
eliminate the overflow.

Alternatively, we could do something like

  prevHeapBlk = 0;
  for (heapBlk = 0; (heapBlk < nblocks) && (prevHeapBlk <= heapBlk);
       heapBlk += pagesPerRange)
  {
    ...
    prevHeapBlk = heapBlk;
  }


regards

-- 
Tomas Vondra
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company