v1-0001-Fix-CLUSTER-progress-reporting-of-number-of-block.patch

From 12f7617f4999b7bf85cc2c9ef4c3b96aac3b26e8 Mon Sep 17 00:00:00 2001
From: Matthias van de Meent <boekewurm@gmail.com>
Date: Fri, 20 Nov 2020 16:23:59 +0100
Subject: [PATCH v1] Fix CLUSTER progress reporting of number of blocks
 scanned.

The heapScan need not start at block 0, so heapScan->rs_cblock need not be the
correct value for amount of blocks scanned. A more correct value is
 ((heapScan->rs_cblock - heapScan->rs_startblock + heapScan->rs_nblocks) %
   heapScan->rs_nblocks), as it accounts for the wraparound and the initial
offset of the heapScan.

Signed-off-by: Matthias van de Meent <boekewurm@gmail.com>
---
 src/backend/access/heap/heapam_handler.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/src/backend/access/heap/heapam_handler.c b/src/backend/access/heap/heapam_handler.c
index dcaea7135f..3d0433633b 100644
--- a/src/backend/access/heap/heapam_handler.c
+++ b/src/backend/access/heap/heapam_handler.c
@@ -798,9 +798,17 @@ heapam_relation_copy_for_cluster(Relation OldHeap, Relation NewHeap,
 			/*
 			 * In scan-and-sort mode and also VACUUM FULL, set heap blocks
 			 * scanned
+			 *
+			 * Note that heapScan may start at an offset and wrap around, i.e.
+			 * rs_startblock may be >0, and rs_cblock may end with a number
+			 * below rs_startblock. To prevent showing this wraparound to the
+			 * user, we offset rs_cblock by rs_startblock (modulo rs_nblocks).
 			 */
 			pgstat_progress_update_param(PROGRESS_CLUSTER_HEAP_BLKS_SCANNED,
-										 heapScan->rs_cblock + 1);
+										 (heapScan->rs_cblock +
+										 heapScan->rs_nblocks -
+										 heapScan->rs_startblock
+										 ) % heapScan->rs_nblocks + 1);
 		}
 
 		tuple = ExecFetchSlotHeapTuple(slot, false, NULL);
-- 
2.20.1