Fix autovacuum log output heap truncation issue.

The percentage of blocks from the table value reported by autovacuum log
output (following commit 5100010ee4d) should never exceed 100% because
it describes the state of the table back when lazy_vacuum() was called.
The value could nevertheless exceed 100% in the event of heap relation
truncation.  We failed to compensate for how truncation affects
rel_pages.

Fix the faulty accounting by using the original rel_pages value instead
of the current/final rel_pages value.

Reported-By: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/20210423204306.5osfpkt2ggaedyvy@alap3.anarazel.de

diff --git a/src/backend/access/heap/vacuumlazy.c b/src/backend/access/heap/vacuumlazy.c
index 17519a970fe95b5c46f85be114c7321d3e0bd859..9f1f8e340d9a391ff1e5755c5e27ed7975abb5f6 100644 (file)
--- a/src/backend/access/heap/vacuumlazy.c
+++ b/src/backend/access/heap/vacuumlazy.c
@@ -777,6 +777,8 @@ heap_vacuum_rel(Relation rel, VacuumParams *params,
                                                         (long long) VacuumPageDirty);
                        if (vacrel->rel_pages > 0)
                        {
+                               BlockNumber orig_rel_pages;
+
                                if (vacrel->do_index_vacuuming)
                                {
                                        msgfmt = _(" %u pages from table (%.2f%% of total) had %lld dead item identifiers removed\n");
@@ -795,9 +797,10 @@ heap_vacuum_rel(Relation rel, VacuumParams *params,
                                        else
                                                appendStringInfo(&buf, _("index scan bypassed by failsafe:"));
                                }
+                               orig_rel_pages = vacrel->rel_pages + vacrel->pages_removed;
                                appendStringInfo(&buf, msgfmt,
                                                                 vacrel->lpdead_item_pages,
-                                                                100.0 * vacrel->lpdead_item_pages / vacrel->rel_pages,
+                                                                100.0 * vacrel->lpdead_item_pages / orig_rel_pages,
                                                                 (long long) vacrel->lpdead_items);
                        }
                        for (int i = 0; i < vacrel->nindexes; i++)