Refine the definition of page-level freezing.

Improve comments added by commit 1de58df4 which describe the
lazy_scan_prune "freeze the page" path.  These newly revised comments
are based on suggestions from Jeff Davis.

In passing, remove nearby visibility_cutoff_xid comments left over from
commit 6daeeb1f.

Author: Peter Geoghegan <pg@bowt.ie>
Reviewed-By: Jeff Davis <pgsql@j-davis.com>
Discussion: https://postgr.es/m/ebc857107fe3edd422ef8a65191ca4a8da568b9b.camel@j-davis.com

1 files changed, 8 insertions, 15 deletions

diff --git a/src/include/access/heapam.h b/src/include/access/heapam.h
index 39c009dbf2..4a69b6dd04 100644
--- a/src/include/access/heapam.h
+++ b/src/include/access/heapam.h
@@ -145,16 +145,14 @@ typedef struct HeapPageFreeze
 	/*
 	 * "Freeze" NewRelfrozenXid/NewRelminMxid trackers.
 	 *
-	 * Trackers used when heap_freeze_execute_prepared freezes the page, and
-	 * when page is "nominally frozen", which happens with pages where every
-	 * call to heap_prepare_freeze_tuple produced no usable freeze plan.
-	 *
-	 * "Nominal freezing" enables vacuumlazy.c's approach of setting a page
-	 * all-frozen in the visibility map when every tuple's 'totally_frozen'
-	 * result is true.  That always works in the same way, independent of the
-	 * need to freeze tuples, and without complicating the general rule around
-	 * 'totally_frozen' results (which is that 'totally_frozen' results are
-	 * only to be trusted with a page that goes on to be frozen by caller).
+	 * Trackers used when heap_freeze_execute_prepared freezes, or when there
+	 * are zero freeze plans for a page.  It is always valid for vacuumlazy.c
+	 * to freeze any page, by definition.  This even includes pages that have
+	 * no tuples with storage to consider in the first place.  That way the
+	 * 'totally_frozen' results from heap_prepare_freeze_tuple can always be
+	 * used in the same way, even when no freeze plans need to be executed to
+	 * "freeze the page".  Only the "freeze" path needs to consider the need
+	 * to set pages all-frozen in the visibility map under this scheme.
 	 *
 	 * When we freeze a page, we generally freeze all XIDs < OldestXmin, only
 	 * leaving behind XIDs that are ineligible for freezing, if any.  And so
@@ -178,11 +176,6 @@ typedef struct HeapPageFreeze
 	 * VACUUM scans a page that isn't cleanup locked.  Both code paths are
 	 * based on the same general idea (do less work for this page during the
 	 * ongoing VACUUM, at the cost of having to accept older final values).
-	 *
-	 * When vacuumlazy.c caller decides to do "no freeze" processing, it must
-	 * not go on to set the page all-frozen (setting the page all-visible
-	 * could still be okay).  heap_prepare_freeze_tuple's 'totally_frozen'
-	 * results can only be used on a page that also gets frozen as instructed.
 	 */
 	TransactionId NoFreezePageRelfrozenXid;
 	MultiXactId NoFreezePageRelminMxid;