Avoid marking buffer dirty when VACUUM has no work to do.

When wal_level = 'hot_standby' we touched the last page of the
relation during a VACUUM, even if nothing else had happened.
That would alter the LSN of the last block and set the mtime
of the relation file unnecessarily. Noted by Thom Brown.

diff --git a/src/backend/access/nbtree/nbtpage.c b/src/backend/access/nbtree/nbtpage.c
index f9b3e1feafe62130e250f68b2183cdf5f944d19b..6f6e676ec08791689a049c7b2a556f2670a7a5b4 100644 (file)
--- a/src/backend/access/nbtree/nbtpage.c
+++ b/src/backend/access/nbtree/nbtpage.c
@@ -732,7 +732,7 @@ _bt_page_recyclable(Page page)
  * and so must be scanned anyway during replay. We always write a WAL record
  * for the last block in the index, whether or not it contained any items
  * to be removed. This allows us to scan right up to end of index to
- * ensure correct locking.
+ * ensure correct locking. That is the only time we are called with nitems==0.
  */
 void
 _bt_delitems_vacuum(Relation rel, Buffer buf,
@@ -764,7 +764,8 @@ _bt_delitems_vacuum(Relation rel, Buffer buf,
         */
        opaque->btpo_flags &= ~BTP_HAS_GARBAGE;
 
-       MarkBufferDirty(buf);
+       if (nitems > 0)
+               MarkBufferDirty(buf);
 
        /* XLOG stuff */
        if (RelationNeedsWAL(rel))
@@ -804,8 +805,11 @@ _bt_delitems_vacuum(Relation rel, Buffer buf,
 
                recptr = XLogInsert(RM_BTREE_ID, XLOG_BTREE_VACUUM, rdata);
 
-               PageSetLSN(page, recptr);
-               PageSetTLI(page, ThisTimeLineID);
+               if (nitems > 0)
+               {
+                       PageSetLSN(page, recptr);
+                       PageSetTLI(page, ThisTimeLineID);
+               }
        }
 
        END_CRIT_SECTION();