Save a few cycles during nbtree VACUUM.

Avoid calling RelationGetNumberOfBlocks() unnecessarily in the common
case where there are no deleted but not yet recycled pages to recycle
during a cleanup-only nbtree VACUUM operation.

Follow-up to commit e5d8a999, which (among other things) taught the
"skip full scan" nbtree VACUUM mechanism to only trigger a full index
scan when the absolute number of deleted pages in the index is
considered excessive.

diff --git a/src/backend/access/nbtree/nbtree.c b/src/backend/access/nbtree/nbtree.c
index c70647d6f309e87ad5a8adf0b143cf77f067efbf..2bcc79ac2fac38e1aeca725f3de3698f444b73a7 100644 (file)
--- a/src/backend/access/nbtree/nbtree.c
+++ b/src/backend/access/nbtree/nbtree.c
@@ -825,9 +825,10 @@ _bt_vacuum_needs_cleanup(IndexVacuumInfo *info)
     * calls.  That is, we can end up scanning the entire index without ever
     * placing even 1 of the prev_num_delpages pages in the free space map, at
     * least in certain narrow cases (see nbtree/README section on recycling
-    * deleted pages for details).  This rarely matters in practice.
+    * deleted pages for details).  This rarely comes up in practice.
     */
-   if (prev_num_delpages > RelationGetNumberOfBlocks(info->index) / 20)
+   if (prev_num_delpages > 0 &&
+       prev_num_delpages > RelationGetNumberOfBlocks(info->index) / 20)
        return true;
 
    return false;
@@ -916,17 +917,12 @@ btvacuumcleanup(IndexVacuumInfo *info, IndexBulkDeleteResult *stats)
    }
 
    /*
-    * By here, we know for sure that this VACUUM operation won't be skipping
-    * its btvacuumscan() call.  Maintain num_delpages value in metapage.
-    * This information will be used by _bt_vacuum_needs_cleanup() during
-    * future VACUUM operations that don't need to call btbulkdelete().
+    * Maintain num_delpages value in metapage for _bt_vacuum_needs_cleanup().
     *
     * num_delpages is the number of deleted pages now in the index that were
     * not safe to place in the FSM to be recycled just yet.  We expect that
     * it will almost certainly be possible to place all of these pages in the
-    * FSM during the next VACUUM operation.  _bt_vacuum_needs_cleanup() will
-    * force the next VACUUM to consider this before allowing btvacuumscan()
-    * to be skipped entirely.
+    * FSM during the next VACUUM operation.
     */
    Assert(stats->pages_deleted >= stats->pages_free);
    num_delpages = stats->pages_deleted - stats->pages_free;