Remove unneeded nbtree latestRemovedXid comments.

petergeoghegan · petergeoghegan · commit 895267a32664 · 2021-09-26T20:25:14.000-07:00
Discussing the low level issue of nbtree VACUUM and recovery conflicts in btvacuumpage() now seems inappropriate. The same issue is discussed in nbtxlog.h, as well as in a comment block above _bt_delitems_vacuum(). The comment block made more sense when it was part of a broader discussion of nbtree VACUUM "pin scans". These were removed by commit 9f83468.
diff --git a/src/backend/access/nbtree/nbtree.c b/src/backend/access/nbtree/nbtree.c
@@ -1183,12 +1183,6 @@ btvacuumpage(BTVacState *vstate, BlockNumber scanblkno)
 			opaque->btpo_next < scanblkno)
 			backtrack_to = opaque->btpo_next;
 
-		/*
-		 * When each VACUUM begins, it determines an OldestXmin cutoff value.
-		 * Tuples before the cutoff are removed by VACUUM.  Scan over all
-		 * items to see which ones need to be deleted according to cutoff
-		 * point using callback.
-		 */
 		ndeletable = 0;
 		nupdatable = 0;
 		minoff = P_FIRSTDATAKEY(opaque);
@@ -1197,6 +1191,7 @@ btvacuumpage(BTVacState *vstate, BlockNumber scanblkno)
 		nhtidslive = 0;
 		if (callback)
 		{
+			/* btbulkdelete callback tells us what to delete (or update) */
 			for (offnum = minoff;
 				 offnum <= maxoff;
 				 offnum = OffsetNumberNext(offnum))
@@ -1206,26 +1201,6 @@ btvacuumpage(BTVacState *vstate, BlockNumber scanblkno)
 				itup = (IndexTuple) PageGetItem(page,
 												PageGetItemId(page, offnum));
 
-				/*
-				 * Hot Standby assumes that it's okay that XLOG_BTREE_VACUUM
-				 * records do not produce their own conflicts.  This is safe
-				 * as long as the callback function only considers whether the
-				 * index tuple refers to pre-cutoff heap tuples that were
-				 * certainly already pruned away during VACUUM's initial heap
-				 * scan by the time we get here. (heapam's XLOG_HEAP2_PRUNE
-				 * records produce conflicts using a latestRemovedXid value
-				 * for the pointed-to heap tuples, so there is no need to
-				 * produce our own conflict now.)
-				 *
-				 * Backends with snapshots acquired after a VACUUM starts but
-				 * before it finishes could have visibility cutoff with a
-				 * later xid than VACUUM's OldestXmin cutoff.  These backends
-				 * might happen to opportunistically mark some index tuples
-				 * LP_DEAD before we reach them, even though they may be after
-				 * our cutoff.  We don't try to kill these "extra" index
-				 * tuples in _bt_delitems_vacuum().  This keep things simple,
-				 * and allows us to always avoid generating our own conflicts.
-				 */
 				Assert(!BTreeTupleIsPivot(itup));
 				if (!BTreeTupleIsPosting(itup))
 				{
