On 2020/11/25 0:25, Matthias van de Meent wrote:
On Tue, 24 Nov 2020 at 15:05, Fujii Masao <masao.fu...@oss.nttdata.com> wrote:
On 2020/11/21 2:32, Matthias van de Meent wrote:
Hi,
The pg_stat_progress_cluster view can report incorrect
heap_blks_scanned values when synchronize_seqscans is enabled, because
it allows the sequential heap scan to not start at block 0. This can
result in wraparounds in the heap_blks_scanned column when the table
scan wraps around, and starting the next phase with heap_blks_scanned
!= heap_blks_total. This issue was introduced with the
pg_stat_progress_cluster view.
Good catch! I agree that this is a bug.
The attached patch fixes the issue by accounting for a non-0
heapScan->rs_startblock and calculating the correct number with a
non-0 heapScan->rs_startblock in mind.
Thanks for the patch! It basically looks good to me.
Thanks for the feedback!
It's a bit waste of cycles to calculate and update the number of scanned
blocks every cycles. So I'm inclined to change the code as follows.
Thought?
+ BlockNumber prev_cblock = InvalidBlockNumber;
<snip>
+ if (prev_cblock != heapScan->rs_cblock)
+ {
+
pgstat_progress_update_param(PROGRESS_CLUSTER_HEAP_BLKS_SCANNED,
+
(heapScan->rs_cblock +
+
heapScan->rs_nblocks -
+
heapScan->rs_startblock
+
) % heapScan->rs_nblocks + 1);
+ prev_cblock = heapScan->rs_cblock;
+ }
That seems quite reasonable.
I noticed that with my proposed patch it is still possible to go to
the next phase while heap_blks_scanned != heap_blks_total. This can
happen when the final heap pages contain only dead tuples, so no tuple
is returned from the last heap page(s) of the scan. As the
heapScan->rs_cblock is set to InvalidBlockNumber when the scan is
finished (see heapam.c#1060-1072), I think it would be correct to set
heap_blks_scanned to heapScan->rs_nblocks at the end of the scan
instead.
Thanks for updating the patch!
Please let me check my understanding about this. I was thinking that even
when the last page contains only dead tuples, table_scan_getnextslot()
returns the last page (because SnapshotAny is used) and heap_blks_scanned
is incremented properly. And then, heapam_relation_copy_for_cluster()
handles all the dead tuples in that page. No?
Regards,
--
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION
