Add options to control whether VACUUM runs vac_update_datfrozenxid.

VACUUM normally ends by running vac_update_datfrozenxid(), which
requires a scan of pg_class.  Therefore, if one attempts to vacuum a
database one table at a time --- as vacuumdb has done since v12 ---
we will spend O(N^2) time in vac_update_datfrozenxid().  That causes
serious performance problems in databases with tens of thousands of
tables, and indeed the effect is measurable with only a few hundred.
To add insult to injury, only one process can run
vac_update_datfrozenxid at the same time per DB, so this behavior
largely defeats vacuumdb's -j option.

Hence, invent options SKIP_DATABASE_STATS and ONLY_DATABASE_STATS
to allow applications to postpone vac_update_datfrozenxid() until the
end of a series of VACUUM requests, and teach vacuumdb to use them.

Per bug #17717 from Gunnar L.  Sadly, this answer doesn't seem
like something we'd consider back-patching, so the performance
problem will remain in v12-v15.

Tom Lane and Nathan Bossart

Discussion: https://postgr.es/m/17717-6c50eb1c7d23a886@postgresql.org

doc/src/sgml/ref/vacuum.sgml

@@ -36,6 +36,8 @@ VACUUM [ FULL ] [ FREEZE ] [ VERBOSE ] [ ANALYZE ] [ <replaceable class="paramet
     PROCESS_TOAST [ <replaceable class="parameter">boolean</replaceable> ]
     TRUNCATE [ <replaceable class="parameter">boolean</replaceable> ]
     PARALLEL <replaceable class="parameter">integer</replaceable>
+    SKIP_DATABASE_STATS [ <replaceable class="parameter">boolean</replaceable> ]
+    ONLY_DATABASE_STATS [ <replaceable class="parameter">boolean</replaceable> ]
 
 <phrase>and <replaceable class="parameter">table_and_columns</replaceable> is:</phrase>
 
@@ -295,6 +297,41 @@ VACUUM [ FULL ] [ FREEZE ] [ VERBOSE ] [ ANALYZE ] [ <replaceable class="paramet
     </listitem>
    </varlistentry>
 
+   <varlistentry>
+    <term><literal>SKIP_DATABASE_STATS</literal></term>
+    <listitem>
+     <para>
+      Specifies that <command>VACUUM</command> should skip updating the
+      database-wide statistics about oldest unfrozen XIDs.  Normally
+      <command>VACUUM</command> will update these statistics once at the
+      end of the command.  However, this can take awhile in a database
+      with a very large number of tables, and it will accomplish nothing
+      unless the table that had contained the oldest unfrozen XID was
+      among those vacuumed.  Moreover, if multiple <command>VACUUM</command>
+      commands are issued in parallel, only one of them can update the
+      database-wide statistics at a time.  Therefore, if an application
+      intends to issue a series of many <command>VACUUM</command>
+      commands, it can be helpful to set this option in all but the last
+      such command; or set it in all the commands and separately
+      issue <literal>VACUUM (ONLY_DATABASE_STATS)</literal> afterwards.
+     </para>
+    </listitem>
+   </varlistentry>
+
+   <varlistentry>
+    <term><literal>ONLY_DATABASE_STATS</literal></term>
+    <listitem>
+     <para>
+      Specifies that <command>VACUUM</command> should do nothing except
+      update the database-wide statistics about oldest unfrozen XIDs.
+      When this option is specified,
+      the <replaceable class="parameter">table_and_columns</replaceable>
+      list must be empty, and no other option may be enabled
+      except <literal>VERBOSE</literal>.
+     </para>
+    </listitem>
+   </varlistentry>
+
    <varlistentry>
     <term><replaceable class="parameter">boolean</replaceable></term>
     <listitem>

src/backend/commands/vacuum.c

@@ -114,6 +114,8 @@ ExecVacuum(ParseState *pstate, VacuumStmt *vacstmt, bool isTopLevel)
 	bool		full = false;
 	bool		disable_page_skipping = false;
 	bool		process_toast = true;
+	bool		skip_database_stats = false;
+	bool		only_database_stats = false;
 	ListCell   *lc;
 
 	/* index_cleanup and truncate values unspecified for now */
@@ -200,6 +202,10 @@ ExecVacuum(ParseState *pstate, VacuumStmt *vacstmt, bool isTopLevel)
 					params.nworkers = nworkers;
 			}
 		}
+		else if (strcmp(opt->defname, "skip_database_stats") == 0)
+			skip_database_stats = defGetBoolean(opt);
+		else if (strcmp(opt->defname, "only_database_stats") == 0)
+			only_database_stats = defGetBoolean(opt);
 		else
 			ereport(ERROR,
 					(errcode(ERRCODE_SYNTAX_ERROR),
@@ -216,7 +222,9 @@ ExecVacuum(ParseState *pstate, VacuumStmt *vacstmt, bool isTopLevel)
 		(freeze ? VACOPT_FREEZE : 0) |
 		(full ? VACOPT_FULL : 0) |
 		(disable_page_skipping ? VACOPT_DISABLE_PAGE_SKIPPING : 0) |
-		(process_toast ? VACOPT_PROCESS_TOAST : 0);
+		(process_toast ? VACOPT_PROCESS_TOAST : 0) |
+		(skip_database_stats ? VACOPT_SKIP_DATABASE_STATS : 0) |
+		(only_database_stats ? VACOPT_ONLY_DATABASE_STATS : 0);
 
 	/* sanity checks on options */
 	Assert(params.options & (VACOPT_VACUUM | VACOPT_ANALYZE));
@@ -349,6 +357,24 @@ vacuum(List *relations, VacuumParams *params,
 				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
 				 errmsg("PROCESS_TOAST required with VACUUM FULL")));
 
+	/* sanity check for ONLY_DATABASE_STATS */
+	if (params->options & VACOPT_ONLY_DATABASE_STATS)
+	{
+		Assert(params->options & VACOPT_VACUUM);
+		if (relations != NIL)
+			ereport(ERROR,
+					(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+					 errmsg("ONLY_DATABASE_STATS cannot be specified with a list of tables")));
+		/* don't require people to turn off PROCESS_TOAST explicitly */
+		if (params->options & ~(VACOPT_VACUUM |
+								VACOPT_VERBOSE |
+								VACOPT_PROCESS_TOAST |
+								VACOPT_ONLY_DATABASE_STATS))
+			ereport(ERROR,
+					(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+					 errmsg("ONLY_DATABASE_STATS cannot be specified with other VACUUM options")));
+	}
+
 	/*
 	 * Create special memory context for cross-transaction storage.
 	 *
@@ -376,7 +402,12 @@ vacuum(List *relations, VacuumParams *params,
 	 * Build list of relation(s) to process, putting any new data in
 	 * vac_context for safekeeping.
 	 */
-	if (relations != NIL)
+	if (params->options & VACOPT_ONLY_DATABASE_STATS)
+	{
+		/* We don't process any tables in this case */
+		Assert(relations == NIL);
+	}
+	else if (relations != NIL)
 	{
 		List	   *newrels = NIL;
 		ListCell   *lc;
@@ -528,11 +559,11 @@ vacuum(List *relations, VacuumParams *params,
 		StartTransactionCommand();
 	}
 
-	if ((params->options & VACOPT_VACUUM) && !IsAutoVacuumWorkerProcess())
+	if ((params->options & VACOPT_VACUUM) &&
+		!(params->options & VACOPT_SKIP_DATABASE_STATS))
 	{
 		/*
 		 * Update pg_database.datfrozenxid, and truncate pg_xact if possible.
-		 * (autovacuum.c does this for itself.)
 		 */
 		vac_update_datfrozenxid();
 	}
@@ -560,13 +591,14 @@ vacuum_is_permitted_for_relation(Oid relid, Form_pg_class reltuple,
 
 	Assert((options & (VACOPT_VACUUM | VACOPT_ANALYZE)) != 0);
 
-	/*
+	/*----------
 	 * A role has privileges to vacuum or analyze the relation if any of the
 	 * following are true:
 	 *   - the role is a superuser
 	 *   - the role owns the relation
 	 *   - the role owns the current database and the relation is not shared
 	 *   - the role has been granted the MAINTAIN privilege on the relation
+	 *----------
 	 */
 	if (object_ownercheck(RelationRelationId, relid, GetUserId()) ||
 		(object_ownercheck(DatabaseRelationId, MyDatabaseId, GetUserId()) && !reltuple->relisshared) ||

src/backend/postmaster/autovacuum.c

@@ -2854,8 +2854,13 @@ table_recheck_autovac(Oid relid, HTAB *table_toast_map,
 		tab->at_relid = relid;
 		tab->at_sharedrel = classForm->relisshared;
 
-		/* Note that this skips toast relations */
+		/*
-		tab->at_params.options = (dovacuum ? VACOPT_VACUUM : 0) |
+		 * Select VACUUM options.  Note we don't say VACOPT_PROCESS_TOAST, so
+		 * that vacuum() skips toast relations.  Also note we tell vacuum() to
+		 * skip vac_update_datfrozenxid(); we'll do that separately.
+		 */
+		tab->at_params.options =
+			(dovacuum ? (VACOPT_VACUUM | VACOPT_SKIP_DATABASE_STATS) : 0) |
 			(doanalyze ? VACOPT_ANALYZE : 0) |
 			(!wraparound ? VACOPT_SKIP_LOCKED : 0);
 

src/bin/psql/tab-complete.c

@@ -4598,8 +4598,9 @@ psql_completion(const char *text, int start, int end)
 			COMPLETE_WITH("FULL", "FREEZE", "ANALYZE", "VERBOSE",
 						  "DISABLE_PAGE_SKIPPING", "SKIP_LOCKED",
 						  "INDEX_CLEANUP", "PROCESS_TOAST",
-						  "TRUNCATE", "PARALLEL");
+						  "TRUNCATE", "PARALLEL", "SKIP_DATABASE_STATS",
-		else if (TailMatches("FULL|FREEZE|ANALYZE|VERBOSE|DISABLE_PAGE_SKIPPING|SKIP_LOCKED|PROCESS_TOAST|TRUNCATE"))
+						  "ONLY_DATABASE_STATS");
+		else if (TailMatches("FULL|FREEZE|ANALYZE|VERBOSE|DISABLE_PAGE_SKIPPING|SKIP_LOCKED|PROCESS_TOAST|TRUNCATE|SKIP_DATABASE_STATS|ONLY_DATABASE_STATS"))
 			COMPLETE_WITH("ON", "OFF");
 		else if (TailMatches("INDEX_CLEANUP"))
 			COMPLETE_WITH("AUTO", "ON", "OFF");

src/bin/scripts/t/100_vacuumdb.pl

@@ -22,15 +22,15 @@ $node->issues_sql_like(
 	'SQL VACUUM run');
 $node->issues_sql_like(
 	[ 'vacuumdb', '-f', 'postgres' ],
-	qr/statement: VACUUM \(FULL\).*;/,
+	qr/statement: VACUUM \(SKIP_DATABASE_STATS, FULL\).*;/,
 	'vacuumdb -f');
 $node->issues_sql_like(
 	[ 'vacuumdb', '-F', 'postgres' ],
-	qr/statement: VACUUM \(FREEZE\).*;/,
+	qr/statement: VACUUM \(SKIP_DATABASE_STATS, FREEZE\).*;/,
 	'vacuumdb -F');
 $node->issues_sql_like(
 	[ 'vacuumdb', '-zj2', 'postgres' ],
-	qr/statement: VACUUM \(ANALYZE\).*;/,
+	qr/statement: VACUUM \(SKIP_DATABASE_STATS, ANALYZE\).*;/,
 	'vacuumdb -zj2');
 $node->issues_sql_like(
 	[ 'vacuumdb', '-Z', 'postgres' ],
@@ -38,11 +38,11 @@ $node->issues_sql_like(
 	'vacuumdb -Z');
 $node->issues_sql_like(
 	[ 'vacuumdb', '--disable-page-skipping', 'postgres' ],
-	qr/statement: VACUUM \(DISABLE_PAGE_SKIPPING\).*;/,
+	qr/statement: VACUUM \(DISABLE_PAGE_SKIPPING, SKIP_DATABASE_STATS\).*;/,
 	'vacuumdb --disable-page-skipping');
 $node->issues_sql_like(
 	[ 'vacuumdb', '--skip-locked', 'postgres' ],
-	qr/statement: VACUUM \(SKIP_LOCKED\).*;/,
+	qr/statement: VACUUM \(SKIP_DATABASE_STATS, SKIP_LOCKED\).*;/,
 	'vacuumdb --skip-locked');
 $node->issues_sql_like(
 	[ 'vacuumdb', '--skip-locked', '--analyze-only', 'postgres' ],
@@ -53,32 +53,32 @@ $node->command_fails(
 	'--analyze-only and --disable-page-skipping specified together');
 $node->issues_sql_like(
 	[ 'vacuumdb', '--no-index-cleanup', 'postgres' ],
-	qr/statement: VACUUM \(INDEX_CLEANUP FALSE\).*;/,
+	qr/statement: VACUUM \(INDEX_CLEANUP FALSE, SKIP_DATABASE_STATS\).*;/,
 	'vacuumdb --no-index-cleanup');
 $node->command_fails(
 	[ 'vacuumdb', '--analyze-only', '--no-index-cleanup', 'postgres' ],
 	'--analyze-only and --no-index-cleanup specified together');
 $node->issues_sql_like(
 	[ 'vacuumdb', '--no-truncate', 'postgres' ],
-	qr/statement: VACUUM \(TRUNCATE FALSE\).*;/,
+	qr/statement: VACUUM \(TRUNCATE FALSE, SKIP_DATABASE_STATS\).*;/,
 	'vacuumdb --no-truncate');
 $node->command_fails(
 	[ 'vacuumdb', '--analyze-only', '--no-truncate', 'postgres' ],
 	'--analyze-only and --no-truncate specified together');
 $node->issues_sql_like(
 	[ 'vacuumdb', '--no-process-toast', 'postgres' ],
-	qr/statement: VACUUM \(PROCESS_TOAST FALSE\).*;/,
+	qr/statement: VACUUM \(PROCESS_TOAST FALSE, SKIP_DATABASE_STATS\).*;/,
 	'vacuumdb --no-process-toast');
 $node->command_fails(
 	[ 'vacuumdb', '--analyze-only', '--no-process-toast', 'postgres' ],
 	'--analyze-only and --no-process-toast specified together');
 $node->issues_sql_like(
 	[ 'vacuumdb', '-P', 2, 'postgres' ],
-	qr/statement: VACUUM \(PARALLEL 2\).*;/,
+	qr/statement: VACUUM \(SKIP_DATABASE_STATS, PARALLEL 2\).*;/,
 	'vacuumdb -P 2');
 $node->issues_sql_like(
 	[ 'vacuumdb', '-P', 0, 'postgres' ],
-	qr/statement: VACUUM \(PARALLEL 0\).*;/,
+	qr/statement: VACUUM \(SKIP_DATABASE_STATS, PARALLEL 0\).*;/,
 	'vacuumdb -P 0');
 $node->command_ok([qw(vacuumdb -Z --table=pg_am dbname=template1)],
 	'vacuumdb with connection string');
@@ -119,7 +119,7 @@ $node->command_fails([ 'vacuumdb', '-P', -1, 'postgres' ],
 	'negative parallel degree');
 $node->issues_sql_like(
 	[ 'vacuumdb', '--analyze', '--table', 'vactable(a, b)', 'postgres' ],
-	qr/statement: VACUUM \(ANALYZE\) public.vactable\(a, b\);/,
+	qr/statement: VACUUM \(SKIP_DATABASE_STATS, ANALYZE\) public.vactable\(a, b\);/,
 	'vacuumdb --analyze with complete column list');
 $node->issues_sql_like(
 	[ 'vacuumdb', '--analyze-only', '--table', 'vactable(b)', 'postgres' ],
@@ -150,7 +150,7 @@ $node->issues_sql_like(
 	'vacuumdb --table --min-xid-age');
 $node->issues_sql_like(
 	[ 'vacuumdb', '--schema', '"Foo"', 'postgres' ],
-	qr/VACUUM "Foo".bar/,
+	qr/VACUUM \(SKIP_DATABASE_STATS\) "Foo".bar/,
 	'vacuumdb --schema');
 $node->issues_sql_like(
 	[ 'vacuumdb', '--exclude-schema', '"Foo"', 'postgres' ],

src/bin/scripts/vacuumdb.c

@@ -44,6 +44,7 @@ typedef struct vacuumingOptions
 	bool		force_index_cleanup;
 	bool		do_truncate;
 	bool		process_toast;
+	bool		skip_database_stats;
 } vacuumingOptions;
 
 /* object filter options */
@@ -533,6 +534,9 @@ vacuum_one_database(ConnParams *cparams,
 		pg_fatal("cannot use the \"%s\" option on server versions older than PostgreSQL %s",
 				 "--parallel", "13");
 
+	/* skip_database_stats is used automatically if server supports it */
+	vacopts->skip_database_stats = (PQserverVersion(conn) >= 160000);
+
 	if (!quiet)
 	{
 		if (stage != ANALYZE_NO_STAGE)
@@ -790,7 +794,29 @@ vacuum_one_database(ConnParams *cparams,
 	} while (cell != NULL);
 
 	if (!ParallelSlotsWaitCompletion(sa))
+	{
 		failed = true;
+		goto finish;
+	}
+
+	/* If we used SKIP_DATABASE_STATS, mop up with ONLY_DATABASE_STATS */
+	if (vacopts->skip_database_stats && stage == ANALYZE_NO_STAGE)
+	{
+		const char *cmd = "VACUUM (ONLY_DATABASE_STATS);";
+		ParallelSlot *free_slot = ParallelSlotsGetIdle(sa, NULL);
+
+		if (!free_slot)
+		{
+			failed = true;
+			goto finish;
+		}
+
+		ParallelSlotSetHandler(free_slot, TableCommandResultHandler, NULL);
+		run_vacuum_command(free_slot->connection, cmd, echo, NULL);
+
+		if (!ParallelSlotsWaitCompletion(sa))
+			failed = true;
+	}
 
 finish:
 	ParallelSlotsTerminate(sa);
@@ -957,6 +983,13 @@ prepare_vacuum_command(PQExpBuffer sql, int serverVersion,
 				appendPQExpBuffer(sql, "%sPROCESS_TOAST FALSE", sep);
 				sep = comma;
 			}
+			if (vacopts->skip_database_stats)
+			{
+				/* SKIP_DATABASE_STATS is supported since v16 */
+				Assert(serverVersion >= 160000);
+				appendPQExpBuffer(sql, "%sSKIP_DATABASE_STATS", sep);
+				sep = comma;
+			}
 			if (vacopts->skip_locked)
 			{
 				/* SKIP_LOCKED is supported since v12 */

src/fe_utils/parallel_slot.c

@@ -475,6 +475,9 @@ ParallelSlotsWaitCompletion(ParallelSlotArray *sa)
 			continue;
 		if (!consumeQueryResult(&sa->slots[i]))
 			return false;
+		/* Mark connection as idle */
+		sa->slots[i].inUse = false;
+		ParallelSlotClearHandler(&sa->slots[i]);
 	}
 
 	return true;

src/include/commands/vacuum.h

@@ -188,6 +188,8 @@ typedef struct VacAttrStats
 #define VACOPT_SKIP_LOCKED 0x20 /* skip if cannot get lock */
 #define VACOPT_PROCESS_TOAST 0x40	/* process the TOAST table, if any */
 #define VACOPT_DISABLE_PAGE_SKIPPING 0x80	/* don't skip any pages */
+#define VACOPT_SKIP_DATABASE_STATS 0x100	/* skip vac_update_datfrozenxid() */
+#define VACOPT_ONLY_DATABASE_STATS 0x200	/* only vac_update_datfrozenxid() */
 
 /*
  * Values used by index_cleanup and truncate params.

src/test/regress/expected/vacuum.out

@@ -282,6 +282,12 @@ ALTER TABLE vactst ALTER COLUMN t SET STORAGE EXTERNAL;
 VACUUM (PROCESS_TOAST FALSE) vactst;
 VACUUM (PROCESS_TOAST FALSE, FULL) vactst;
 ERROR:  PROCESS_TOAST required with VACUUM FULL
+-- SKIP_DATABASE_STATS option
+VACUUM (SKIP_DATABASE_STATS) vactst;
+-- ONLY_DATABASE_STATS option
+VACUUM (ONLY_DATABASE_STATS);
+VACUUM (ONLY_DATABASE_STATS) vactst;  -- error
+ERROR:  ONLY_DATABASE_STATS cannot be specified with a list of tables
 DROP TABLE vaccluster;
 DROP TABLE vactst;
 DROP TABLE vacparted;

src/test/regress/sql/vacuum.sql

@@ -237,6 +237,13 @@ ALTER TABLE vactst ALTER COLUMN t SET STORAGE EXTERNAL;
 VACUUM (PROCESS_TOAST FALSE) vactst;
 VACUUM (PROCESS_TOAST FALSE, FULL) vactst;
 
+-- SKIP_DATABASE_STATS option
+VACUUM (SKIP_DATABASE_STATS) vactst;
+
+-- ONLY_DATABASE_STATS option
+VACUUM (ONLY_DATABASE_STATS);
+VACUUM (ONLY_DATABASE_STATS) vactst;  -- error
+
 DROP TABLE vaccluster;
 DROP TABLE vactst;
 DROP TABLE vacparted;