VACUUM VERBOSE: Count "newly deleted" index pages.

Teach VACUUM VERBOSE to report on pages deleted by the _current_ VACUUM
operation -- these are newly deleted pages.  VACUUM VERBOSE continues to
report on the total number of deleted pages in the entire index (no
change there).  The former is a subset of the latter.

The distinction between each category of deleted index page only arises
with index AMs where page deletion is supported and is decoupled from
page recycling for performance reasons.

This is follow-up work to commit e5d8a999, which made nbtree store
64-bit XIDs (not 32-bit XIDs) in pages at the point at which they're
deleted.  Note that the btm_last_cleanup_num_delpages metapage field
added by that commit usually gets set to pages_newly_deleted.  The
exceptions (the scenarios in which they're not equal) all seem to be
tricky cases for the implementation (of page deletion and recycling) in
general.

Author: Peter Geoghegan <pg@bowt.ie>
Discussion: https://postgr.es/m/CAH2-WznpdHvujGUwYZ8sihX%3Dd5u-tRYhi-F4wnV2uN2zHpMUXw%40mail.gmail.com

1 files changed, 16 insertions, 3 deletions

diff --git a/src/backend/access/gist/gistvacuum.c b/src/backend/access/gist/gistvacuum.c
index ddecb8ab18e..0663193531a 100644
--- a/src/backend/access/gist/gistvacuum.c
+++ b/src/backend/access/gist/gistvacuum.c
@@ -133,9 +133,21 @@ gistvacuumscan(IndexVacuumInfo *info, IndexBulkDeleteResult *stats,
 	MemoryContext oldctx;
 
 	/*
-	 * Reset counts that will be incremented during the scan; needed in case
-	 * of multiple scans during a single VACUUM command.
+	 * Reset fields that track information about the entire index now.  This
+	 * avoids double-counting in the case where a single VACUUM command
+	 * requires multiple scans of the index.
+	 *
+	 * Avoid resetting the tuples_removed and pages_newly_deleted fields here,
+	 * since they track information about the VACUUM command, and so must last
+	 * across each call to gistvacuumscan().
+	 *
+	 * (Note that pages_free is treated as state about the whole index, not
+	 * the current VACUUM.  This is appropriate because RecordFreeIndexPage()
+	 * calls are idempotent, and get repeated for the same deleted pages in
+	 * some scenarios.  The point for us is to track the number of recyclable
+	 * pages in the index at the end of the VACUUM command.)
 	 */
+	stats->num_pages = 0;
 	stats->estimated_count = false;
 	stats->num_index_tuples = 0;
 	stats->pages_deleted = 0;
@@ -281,8 +293,8 @@ restart:
 	{
 		/* Okay to recycle this page */
 		RecordFreeIndexPage(rel, blkno);
-		vstate->stats->pages_free++;
 		vstate->stats->pages_deleted++;
+		vstate->stats->pages_free++;
 	}
 	else if (GistPageIsDeleted(page))
 	{
@@ -636,6 +648,7 @@ gistdeletepage(IndexVacuumInfo *info, IndexBulkDeleteResult *stats,
 	/* mark the page as deleted */
 	MarkBufferDirty(leafBuffer);
 	GistPageSetDeleted(leafPage, txid);
+	stats->pages_newly_deleted++;
 	stats->pages_deleted++;
 
 	/* remove the downlink from the parent */