Re: Can rs_cindex be < 0 for bitmap heap scans?

On Thu, Oct 24, 2024 at 3:45 AM Melanie Plageman <melanieplageman(at)gmail(dot)com>
wrote:

> Hi,
>
> HeapScanDescData->rs_cindex (the current index into the array of
> visible tuples stored in the heap scan descriptor) is used for
> multiple scan types, but for bitmap heap scans, AFAICT, it should
> never be < 0.
>
> As such, I find this test in heapam_scan_bitmap_next_tuple() pretty
> confusing.
>
> if (hscan->rs_cindex < 0 || hscan->rs_cindex >= hscan->rs_ntuples)
>
> I know it seems innocuous, but I find it pretty distracting. Am I
> missing something? Could it somehow be < 0?
>
> If not, I propose removing that part of the if statement like in the
> attached patch.
>
>
You are right it can never be < 0 in this case at least. In fact you don't
need to explicitly set it to 0 in initscan[1], because before
calling heapam_scan_bitmap_next_tuple() we must
call heapam_scan_bitmap_next_block() and this function is initializing this
to 0 (hscan->rs_cindex = 0;). Anyway no object even if you prefer to
initialize in initscan(), just the point is that we are already doing it
for each block before fetching tuples from the block.

[1]
@@ -378,6 +378,7 @@ initscan(HeapScanDesc scan, ScanKey key, bool
keep_startblock)
ItemPointerSetInvalid(&scan->rs_ctup.t_self);
scan->rs_cbuf = InvalidBuffer;
scan->rs_cblock = InvalidBlockNumber;
+ scan->rs_cindex = 0;

--
Regards,
Dilip Kumar
EnterpriseDB: http://www.enterprisedb.com