Code review for b6fb6471.

Reports by Tomas Vondra, Vinayak Pokale, and Aleksander Alekseev.
Patch by Amit Langote.

src/backend/postmaster/pgstat.c

@@ -2861,8 +2861,8 @@ pgstat_report_activity(BackendState state, const char *cmd_str)
 /*-----------
  * pgstat_progress_start_command() -
  *
- * Set st_command in own backend entry.  Also, zero-initialize
- * st_progress_param array.
+ * Set st_progress_command (and st_progress_command_target) in own backend
+ * entry.  Also, zero-initialize st_progress_param array.
  *-----------
  */
 void
@@ -2904,7 +2904,8 @@ pgstat_progress_update_param(int index, int64 val)
 /*-----------
  * pgstat_progress_end_command() -
  *
- * Update index'th member in st_progress_param[] of own backend entry.
+ * Reset st_progress_command (and st_progress_command_target) in own backend
+ * entry.  This signals the end of the command.
  *-----------
  */
 void

src/backend/utils/adt/pgstatfuncs.c

@@ -614,7 +614,7 @@ pg_stat_get_progress_info(PG_FUNCTION_ARGS)
 		else
 		{
 			nulls[2] = true;
-			for (i = 1; i < PGSTAT_NUM_PROGRESS_PARAM + 1; i++)
+			for (i = 0; i < PGSTAT_NUM_PROGRESS_PARAM; i++)
 				nulls[i+3] = true;
 		}