Re: Can rs_cindex be < 0 for bitmap heap scans? - Mailing list pgsql-hackers

On Thu, Dec 19, 2024 at 8:18 AM Melanie Plageman
<melanieplageman@gmail.com> wrote:
> I pushed the straightforward option for now so that it's fixed.

I think this binary search code now has a risk of underflow.  If 'mid'
is calculated as zero, the second 'if' branch will cause 'end' to
underflow.

Maybe we need to do something like below.

--- a/src/backend/access/heap/heapam_handler.c
+++ b/src/backend/access/heap/heapam_handler.c
@@ -2600,7 +2600,11 @@ SampleHeapTupleVisible(TableScanDesc scan, Buffer buffer,
            if (tupoffset == curoffset)
                return true;
            else if (tupoffset < curoffset)
+           {
+               if (mid == 0)
+                   return false;
                end = mid - 1;
+           }
            else
                start = mid + 1;
        }

Alternatively, we can revert 'start' and 'end' to signed int as they
were before.

Thanks
Richard