Add delta base TPS with fix for metrics collection

internal/pgmetrics/collector.go

@@ -193,19 +193,22 @@ func (d *defaultStatsCollector) QueryStats(ctx context.Context, db *sql.DB) (Pos
 			COALESCE(SUM(xact_commit), 0) AS xact_commit,
 			COALESCE(SUM(xact_rollback), 0) AS xact_rollback,
 			COALESCE(SUM(blks_read), 0) AS blks_read,
-			COALESCE(MIN(stats_reset), now()) AS stats_reset
-		FROM pg_stat_database
-		WHERE stats_reset IS NOT NULL;
+			MIN(stats_reset) AS stats_reset
+		FROM pg_stat_database;
 	`
 	var stats PostgresStats
+	var statsReset sql.NullTime
 	err := db.QueryRowContext(ctx, query).Scan(
 		&stats.XactCommit,
 		&stats.XactRollback,
 		&stats.BlksRead,
-		&stats.StatsReset,
+		&statsReset,
 	)
 	if err != nil {
 		return PostgresStats{}, fmt.Errorf("query stats: %w", err)
 	}
+	if statsReset.Valid {
+		stats.StatsReset = statsReset.Time
+	}
 	return stats, nil
 }

test/integration/pgmetrics_test.go

@@ -32,7 +32,7 @@ func setupPostgresContainer(t *testing.T) (*postgres.PostgresContainer, credenti
 	ctx := context.Background()
 
 	pgContainer, err := postgres.Run(ctx,
-		"postgres:16-alpine",
+		"postgres:18-alpine",
 		postgres.WithDatabase(testDatabase),
 		postgres.WithUsername(testUser),
 		postgres.WithPassword(testPassword),
@@ -165,6 +165,44 @@ func TestCollector_Collect_CacheHitRatio(t *testing.T) {
 		"cache hit ratio should be greater than 0 after queries")
 }
 
+func TestCollector_Collect_DeltaBasedTPS(t *testing.T) {
+	_, cred := setupPostgresContainer(t)
+
+	// Generate some initial activity to ensure stats_reset is populated
+	connStr := fmt.Sprintf("host=%s port=%d user=%s password=%s dbname=%s sslmode=disable",
+		cred.Host, cred.Port, cred.Username, *cred.Password, cred.Database)
+	db, err := sql.Open("postgres", connStr)
+	require.NoError(t, err)
+	defer db.Close()
+
+	ctx := context.Background()
+	_, err = db.ExecContext(ctx, "SELECT 1")
+	require.NoError(t, err)
+
+	collector := pgmetrics.New()
+
+	// First collection establishes baseline, returns 0 for rate metrics
+	metrics1, err := collector.Collect(cred)
+	require.NoError(t, err)
+	assert.Equal(t, 0.0, metrics1.TransactionsPerSecond, "first collection should return 0 TPS")
+	assert.Equal(t, 0.0, metrics1.CommittedTxPerSecond, "first collection should return 0 committed TPS")
+
+	// Generate transactions between collections
+	for range 100 {
+		_, err = db.ExecContext(ctx, "SELECT 1")
+		require.NoError(t, err)
+	}
+
+	// PostgreSQL stats flush at minimum 1-second intervals
+	time.Sleep(1 * time.Second)
+
+	// Second collection calculates delta-based TPS
+	metrics2, err := collector.Collect(cred)
+	require.NoError(t, err)
+	assert.Greater(t, metrics2.TransactionsPerSecond, 0.0, "second collection should have TPS > 0")
+	assert.Greater(t, metrics2.CommittedTxPerSecond, 0.0, "second collection should have committed TPS > 0")
+}
+
 func BenchmarkCollector_Collect(b *testing.B) {
 	_, cred := setupPostgresContainer(&testing.T{})
 	setupTestData(&testing.T{}, cred)