Fix initialization of pg_stat_get_lastscan()

A NULL result should be reported when a stats timestamp is set to 0, but
c037471 missed that, leading to a confusing timestamp value after for
example a DML on a freshly-created relation with no scans done on it
yet.

This impacted the following attributes for two system views:
- pg_stat_all_tables.last_idx_scan
- pg_stat_all_tables.last_seq_scan
- pg_stat_all_indexes.last_idx_scan

Reported-by: Robert Treat
Analyzed-by: Peter Eisentraut
Author: Dave Page
Discussion: https://postgr.es/m/CABV9wwPzMfSaz3EfKXXDxKmMprbxwF5r6WPuxqA=5mzRUqfTGg@mail.gmail.com

src/backend/utils/adt/pgstatfuncs.c

@@ -56,12 +56,18 @@ Datum
 pg_stat_get_lastscan(PG_FUNCTION_ARGS)
 {
 	Oid			relid = PG_GETARG_OID(0);
+	TimestampTz result;
 	PgStat_StatTabEntry *tabentry;
 
 	if ((tabentry = pgstat_fetch_stat_tabentry(relid)) == NULL)
+		result = 0;
+	else
+		result = tabentry->lastscan;
+
+	if (result == 0)
 		PG_RETURN_NULL();
 	else
-		PG_RETURN_TIMESTAMPTZ(tabentry->lastscan);
+		PG_RETURN_TIMESTAMPTZ(result);
 }
 
 

src/test/regress/expected/stats.out

@@ -573,6 +573,12 @@ SELECT pg_stat_force_next_flush();
  
 (1 row)
 
+SELECT last_seq_scan, last_idx_scan FROM pg_stat_all_tables WHERE relid = 'test_last_scan'::regclass;
+ last_seq_scan | last_idx_scan 
+---------------+---------------
+               | 
+(1 row)
+
 COMMIT;
 SELECT pg_stat_reset_single_table_counters('test_last_scan'::regclass);
  pg_stat_reset_single_table_counters 

src/test/regress/sql/stats.sql

@@ -303,6 +303,7 @@ BEGIN;
 CREATE TEMPORARY TABLE test_last_scan(idx_col int primary key, noidx_col int);
 INSERT INTO test_last_scan(idx_col, noidx_col) VALUES(1, 1);
 SELECT pg_stat_force_next_flush();
+SELECT last_seq_scan, last_idx_scan FROM pg_stat_all_tables WHERE relid = 'test_last_scan'::regclass;
 COMMIT;
 
 SELECT pg_stat_reset_single_table_counters('test_last_scan'::regclass);