Add db index to log entry timestamp

Makes a small but observable performance improvement to the audit log report

wagtail/core/migrations/0064_log_timestamp_indexes.py

@@ -0,0 +1,23 @@
+# Generated by Django 3.2.5 on 2021-07-20 12:14
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('wagtailcore', '0063_modellogentry'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='modellogentry',
+            name='timestamp',
+            field=models.DateTimeField(db_index=True, verbose_name='timestamp (UTC)'),
+        ),
+        migrations.AlterField(
+            model_name='pagelogentry',
+            name='timestamp',
+            field=models.DateTimeField(db_index=True, verbose_name='timestamp (UTC)'),
+        ),
+    ]

wagtail/core/models/audit_log.py

@@ -89,7 +89,7 @@ class BaseLogEntry(models.Model):
 
     action = models.CharField(max_length=255, blank=True, db_index=True)
     data_json = models.TextField(blank=True)
-    timestamp = models.DateTimeField(verbose_name=_('timestamp (UTC)'))
+    timestamp = models.DateTimeField(verbose_name=_('timestamp (UTC)'), db_index=True)
 
     user = models.ForeignKey(
         settings.AUTH_USER_MODEL,