Fix NULL on long_running_transactions collector (#1230)

When there are no long running transactions, the oldest timestamp will be NULL. This sets the metrics value to 0 (age). Also adds a test for this behavior.

Fixes #1223

Signed-off-by: Joe Adams <github@joeadams.io>

collector/pg_long_running_transactions.go

@@ -15,6 +15,7 @@ package collector
 
 import (
 	"context"
+	"database/sql"
 	"log/slog"
 
 	"github.com/prometheus/client_golang/prometheus"
@@ -50,7 +51,7 @@ var (
 	)
 
 	longRunningTransactionsQuery = `
-	SELECT                                                              
+	SELECT
     COUNT(*) as transactions,
     MAX(EXTRACT(EPOCH FROM clock_timestamp() - pg_stat_activity.xact_start)) AS oldest_timestamp_seconds
 FROM pg_catalog.pg_stat_activity
@@ -72,12 +73,20 @@ func (PGLongRunningTransactionsCollector) Update(ctx context.Context, instance *
 	defer rows.Close()
 
 	for rows.Next() {
-		var transactions, ageInSeconds float64
+		var transactions float64
+		var ageInSeconds sql.NullFloat64
 
 		if err := rows.Scan(&transactions, &ageInSeconds); err != nil {
 			return err
 		}
 
+		// If there are no long running transactions, ageInSeconds will be NULL
+		// so we set it to 0
+		age := 0.0
+		if ageInSeconds.Valid {
+			age = ageInSeconds.Float64
+		}
+
 		ch <- prometheus.MustNewConstMetric(
 			longRunningTransactionsCount,
 			prometheus.GaugeValue,
@@ -86,7 +95,7 @@ func (PGLongRunningTransactionsCollector) Update(ctx context.Context, instance *
 		ch <- prometheus.MustNewConstMetric(
 			longRunningTransactionsAgeInSeconds,
 			prometheus.GaugeValue,
-			ageInSeconds,
+			age,
 		)
 	}
 	if err := rows.Err(); err != nil {