Fix comment on when HOT update is possible.

hlinnaka · hlinnaka · commit 3293330f79af · 2019-05-14T13:06:33.000+03:00
The conditions listed in this comment have changed several times, and at
some point the thing that the "if so" referred to was negated.

The text was OK up to 9.6. It was differently wrong in v10, v11 and
master, so fix in all those versions.
diff --git a/src/backend/access/heap/heapam.c b/src/backend/access/heap/heapam.c
@@ -4217,11 +4217,10 @@ heap_update(Relation relation, ItemPointer otid, HeapTuple newtup,
 	{
 		/*
 		 * Since the new tuple is going into the same page, we might be able
-		 * to do a HOT update.  Check if any of the index columns have been
-		 * changed, or if we have projection functional indexes, check whether
-		 * the old and the new values are the same.   If the page was already
-		 * full, we may have skipped checking for index columns. If so, HOT
-		 * update is possible.
+		 * to do a HOT update.  A HOT update is possible if none of the index
+		 * columns, nor columns used in projection functional indexes, have
+		 * changed.  If the page was already full, we may have skipped
+		 * checking for index columns, and also can't do a HOT update.
 		 */
 		if (hot_attrs_checked
 			&& !bms_overlap(modified_attrs, hot_attrs)
