Fix mesurement of elapsed time during truncating heap in VACUUM.

VACUUM may truncate heap in several batches. The activity report
is logged for each batch, and contains the number of pages in the table
before and after the truncation, and also the elapsed time during
the truncation. Previously the elapsed time reported in each batch was
the total elapsed time since starting the truncation until finishing
each batch. For example, if the truncation was processed dividing into
three batches, the second batch reported the accumulated time elapsed
during both first and second batches. This is strange and confusing
because the number of pages in the table reported together is not
total. Instead, each batch should report the time elapsed during
only that batch.

The cause of this issue was that the resource usage snapshot was
initialized only at the beginning of the truncation and was never
reset later. This commit fixes the issue by changing VACUUM so that
the resource usage snapshot is reset at each batch.

Back-patch to all supported branches.

Reported-by: Tatsuhito Kasahara
Author: Tatsuhito Kasahara
Reviewed-by: Masahiko Sawada, Fujii Masao
Discussion: https://postgr.es/m/CAP0=ZVJsf=NvQuy+QXQZ7B=ZVLoDV_JzsVC1FRsF1G18i3zMGg@mail.gmail.com

1 files changed, 4 insertions, 3 deletions

diff --git a/src/backend/access/heap/vacuumlazy.c b/src/backend/access/heap/vacuumlazy.c
index a3c4a1df3b4..8acb8b8eb69 100644
--- a/src/backend/access/heap/vacuumlazy.c
+++ b/src/backend/access/heap/vacuumlazy.c
@@ -1874,11 +1874,8 @@ lazy_truncate_heap(Relation onerel, LVRelStats *vacrelstats)
 {
 	BlockNumber old_rel_pages = vacrelstats->rel_pages;
 	BlockNumber new_rel_pages;
-	PGRUsage	ru0;
 	int			lock_retry;
 
-	pg_rusage_init(&ru0);
-
 	/* Report that we are now truncating */
 	pgstat_progress_update_param(PROGRESS_VACUUM_PHASE,
 								 PROGRESS_VACUUM_PHASE_TRUNCATE);
@@ -1888,6 +1885,10 @@ lazy_truncate_heap(Relation onerel, LVRelStats *vacrelstats)
 	 */
 	do
 	{
+		PGRUsage	ru0;
+
+		pg_rusage_init(&ru0);
+
 		/*
 		 * We need full exclusive lock on the relation in order to do
 		 * truncation. If we can't get it, give up rather than waiting --- we