Add code comment explaining ins_since_vacuum and aborted inserts

Sami complained that there's a discrepancy between n_mod_since_analyze
and n_ins_since_vacuum, as the former only accounts for committed changes
and the latter tracks committed and aborted inserts.  Nobody seemed
overly concerned that this would cause any concerning issues.  The
repercussions, from what I can tell, are limited to causing an
autovacuum to trigger for inserts sooner than it otherwise might. For
typical ratios of commits to aborts, it's unlikely to ever be noticed.

Fixing things to make it so n_ins_since_vacuum only displays committed
inserts would require an additional field in PgStat_TableCounts, which
does not quite seem worthwhile at this stage.  This commit just adds a
comment with some details to mention that we know about it, which will
hopefully prevent repeat discussions.

Reported-by: Sami Imseih <[email protected]>
Author: David Rowley <[email protected]>
Reviewed-by: Sami Imseih <[email protected]>
Discussion: https://postgr.es/m/CAApHDvpgV3a-R2EGmPOh0L-x3pHbZpM3y4dySWfy+UqUazwDQA@mail.gmail.com

1 files changed, 9 insertions, 0 deletions

diff --git a/src/backend/utils/activity/pgstat_relation.c b/src/backend/utils/activity/pgstat_relation.c
index d64595a165c..eeb2d43cb10 100644
--- a/src/backend/utils/activity/pgstat_relation.c
+++ b/src/backend/utils/activity/pgstat_relation.c
@@ -868,7 +868,16 @@ pgstat_relation_flush_cb(PgStat_EntryRef *entry_ref, bool nowait)
 	tabentry->live_tuples += lstats->counts.delta_live_tuples;
 	tabentry->dead_tuples += lstats->counts.delta_dead_tuples;
 	tabentry->mod_since_analyze += lstats->counts.changed_tuples;
+
+	/*
+	 * Using tuples_inserted to update ins_since_vacuum does mean that we'll
+	 * track aborted inserts too.  This isn't ideal, but otherwise probably
+	 * not worth adding an extra field for.  It may just amount to autovacuums
+	 * triggering for inserts more often than they maybe should, which is
+	 * probably not going to be common enough to be too concerned about here.
+	 */
 	tabentry->ins_since_vacuum += lstats->counts.tuples_inserted;
+
 	tabentry->blocks_fetched += lstats->counts.blocks_fetched;
 	tabentry->blocks_hit += lstats->counts.blocks_hit;