vacuumdb: Add --dry-run.

This new option instructs vacuumdb to print, but not execute, the
VACUUM and ANALYZE commands that would've been sent to the server.

Author: Corey Huinker <corey.huinker@gmail.com>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Reviewed-by: Kirill Reshke <reshkekirill@gmail.com>
Reviewed-by: Álvaro Herrera <alvherre@kurilemu.de>
Discussion: https://postgr.es/m/CADkLM%3DckHkX7Of5SrK7g0LokPUwJ%3Dkk8JU1GXGF5pZ1eBVr0%3DQ%40mail.gmail.com

doc/src/sgml/ref/vacuumdb.sgml

@@ -171,6 +171,16 @@ PostgreSQL documentation
       </listitem>
      </varlistentry>
 
+     <varlistentry>
+      <term><option>--dry-run</option></term>
+      <listitem>
+       <para>
+        Print, but do not execute, the vacuum and analyze commands that would
+        have been sent to the server.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry>
       <term><option>-e</option></term>
       <term><option>--echo</option></term>

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

@@ -169,6 +169,10 @@ $node->issues_sql_like(
 	[ 'vacuumdb', '--schema' => '"Foo"', 'postgres' ],
 	qr/VACUUM \(SKIP_DATABASE_STATS\) "Foo".bar/,
 	'vacuumdb --schema');
+$node->issues_sql_unlike(
+	[ 'vacuumdb', '--schema' => '"Foo"', 'postgres', '--dry-run' ],
+	qr/VACUUM \(SKIP_DATABASE_STATS\) "Foo".bar/,
+	'vacuumdb --dry-run');
 $node->issues_sql_like(
 	[ 'vacuumdb', '--schema' => '"Foo"', '--schema' => '"Bar"', 'postgres' ],
 	qr/VACUUM\ \(SKIP_DATABASE_STATS\)\ "Foo".bar
@@ -241,6 +245,14 @@ $node->safe_psql('postgres', q|
   CREATE TABLE regression_vacuumdb_test AS select generate_series(1, 10) a, generate_series(2, 11) b;
   ALTER TABLE regression_vacuumdb_test ADD COLUMN c INT GENERATED ALWAYS AS (a + b);
 |);
+$node->issues_sql_unlike(
+	[
+		'vacuumdb', '--analyze-only', '--dry-run',
+		'--missing-stats-only', '-t',
+		'regression_vacuumdb_test', 'postgres'
+	],
+	qr/statement:\ ANALYZE/sx,
+	'--missing-stats-only --dry-run');
 $node->issues_sql_like(
 	[
 		'vacuumdb', '--analyze-only',

src/bin/scripts/vacuumdb.c

@@ -59,6 +59,7 @@ main(int argc, char *argv[])
 		{"no-process-main", no_argument, NULL, 12},
 		{"buffer-usage-limit", required_argument, NULL, 13},
 		{"missing-stats-only", no_argument, NULL, 14},
+		{"dry-run", no_argument, NULL, 15},
 		{NULL, 0, NULL, 0}
 	};
 
@@ -207,6 +208,9 @@ main(int argc, char *argv[])
 			case 14:
 				vacopts.missing_stats_only = true;
 				break;
+			case 15:
+				vacopts.dry_run = true;
+				break;
 			default:
 				/* getopt_long already emitted a complaint */
 				pg_log_error_hint("Try \"%s --help\" for more information.", progname);
@@ -303,6 +307,10 @@ main(int argc, char *argv[])
 		pg_fatal("cannot use the \"%s\" option without \"%s\" or \"%s\"",
 				 "missing-stats-only", "analyze-only", "analyze-in-stages");
 
+	if (vacopts.dry_run && !vacopts.quiet)
+		pg_log_info("Executing in dry-run mode.\n"
+					"No commands will be sent to the server.");
+
 	ret = vacuuming_main(&cparams, dbname, maintenance_db, &vacopts,
 						 &objects, tbl_count,
 						 concurrentCons,
@@ -345,6 +353,7 @@ help(const char *progname)
 	printf(_("      --buffer-usage-limit=SIZE   size of ring buffer used for vacuum\n"));
 	printf(_("  -d, --dbname=DBNAME             database to vacuum\n"));
 	printf(_("      --disable-page-skipping     disable all page-skipping behavior\n"));
+	printf(_("      --dry-run                   show the commands that would be sent to the server\n"));
 	printf(_("  -e, --echo                      show the commands being sent to the server\n"));
 	printf(_("  -f, --full                      do full vacuuming\n"));
 	printf(_("  -F, --freeze                    freeze row transaction information\n"));

src/bin/scripts/vacuuming.c

@@ -42,8 +42,9 @@ static SimpleStringList *retrieve_objects(PGconn *conn,
 static void free_retrieved_objects(SimpleStringList *list);
 static void prepare_vacuum_command(PGconn *conn, PQExpBuffer sql,
 								   vacuumingOptions *vacopts, const char *table);
-static void run_vacuum_command(PGconn *conn, vacuumingOptions *vacopts,
-							   const char *sql, const char *table);
+static void run_vacuum_command(ParallelSlot *free_slot,
+							   vacuumingOptions *vacopts, const char *sql,
+							   const char *table);
 
 /*
  * Executes vacuum/analyze as indicated.  Returns 0 if the plan is carried
@@ -340,6 +341,10 @@ vacuum_one_database(ConnParams *cparams,
 	if (vacopts->mode == MODE_ANALYZE_IN_STAGES)
 	{
 		initcmd = stage_commands[stage];
+
+		if (vacopts->dry_run)
+			printf("%s\n", initcmd);
+		else
 			executeCommand(conn, initcmd, vacopts->echo);
 	}
 	else
@@ -383,7 +388,7 @@ vacuum_one_database(ConnParams *cparams,
 		 * through ParallelSlotsGetIdle.
 		 */
 		ParallelSlotSetHandler(free_slot, TableCommandResultHandler, NULL);
-		run_vacuum_command(free_slot->connection, vacopts, sql.data, tabname);
+		run_vacuum_command(free_slot, vacopts, sql.data, tabname);
 
 		cell = cell->next;
 	} while (cell != NULL);
@@ -407,7 +412,7 @@ vacuum_one_database(ConnParams *cparams,
 		}
 
 		ParallelSlotSetHandler(free_slot, TableCommandResultHandler, NULL);
-		run_vacuum_command(free_slot->connection, vacopts, cmd, NULL);
+		run_vacuum_command(free_slot, vacopts, cmd, NULL);
 
 		if (!ParallelSlotsWaitCompletion(sa))
 			ret = EXIT_FAILURE; /* error already reported by handler */
@@ -995,19 +1000,24 @@ prepare_vacuum_command(PGconn *conn, PQExpBuffer sql,
 
 /*
  * Send a vacuum/analyze command to the server, returning after sending the
- * command.
+ * command.  If dry_run is true, the command is printed but not sent to the
+ * server.
  *
  * Any errors during command execution are reported to stderr.
  */
 static void
-run_vacuum_command(PGconn *conn, vacuumingOptions *vacopts,
+run_vacuum_command(ParallelSlot *free_slot, vacuumingOptions *vacopts,
 				   const char *sql, const char *table)
 {
-	bool		status;
+	bool		status = true;
+	PGconn	   *conn = free_slot->connection;
 
-	if (vacopts->echo)
+	if (vacopts->echo || vacopts->dry_run)
 		printf("%s\n", sql);
 
+	if (vacopts->dry_run)
+		ParallelSlotSetIdle(free_slot);
+	else
 		status = PQsendQuery(conn, sql) == 1;
 
 	if (!status)

src/bin/scripts/vacuuming.h

@@ -53,6 +53,7 @@ typedef struct vacuumingOptions
 	bool		missing_stats_only;
 	bool		echo;
 	bool		quiet;
+	bool		dry_run;
 } vacuumingOptions;
 
 /* Valid values for vacuumingOptions->objfilter */