Further SQL optimizations

src/pretix/base/models/items.py

@@ -582,7 +582,7 @@ class Quota(LoggedModel):
             Q(redeemed=False) &
             Q(Q(valid_until__isnull=True) | Q(valid_until__gte=now_dt)) &
             Q(Q(self._position_lookup) | Q(quota=self))
-        ).distinct().count()
+        ).values('id').distinct().count()
 
     def count_in_cart(self, now_dt: datetime=None) -> int:
         from pretix.base.models import CartPosition
@@ -595,21 +595,22 @@ class Quota(LoggedModel):
                 & Q(Q(voucher__valid_until__isnull=True) | Q(voucher__valid_until__gte=now_dt))
             ) &
             self._position_lookup
-        ).distinct().count()
+        ).values('id').distinct().count()
 
     def count_pending_orders(self) -> dict:
         from pretix.base.models import Order, OrderPosition
 
+        # This query has beeen benchmarked against a Count('id', distinct=True) aggregate and won by a small margin.
         return OrderPosition.objects.filter(
             self._position_lookup, order__status=Order.STATUS_PENDING,
-        ).distinct().count()
+        ).values('id').distinct().count()
 
     def count_paid_orders(self):
         from pretix.base.models import Order, OrderPosition
 
         return OrderPosition.objects.filter(
             self._position_lookup, order__status=Order.STATUS_PAID
-        ).distinct().count()
+        ).values('id').distinct().count()
 
     @cached_property
     def _position_lookup(self) -> Q:

src/pretix/base/models/orders.py

@@ -515,7 +515,8 @@ class CartPosition(AbstractPosition):
         auto_now_add=True
     )
     expires = models.DateTimeField(
-        verbose_name=_("Expiration date")
+        verbose_name=_("Expiration date"),
+        db_index=True
     )
 
     class Meta:

src/pretix/base/models/vouchers.py

@@ -70,10 +70,11 @@ class Voucher(LoggedModel):
     )
     redeemed = models.BooleanField(
         verbose_name=_("Redeemed"),
-        default=False
+        default=False,
+        db_index=True
     )
     valid_until = models.DateTimeField(
-        blank=True, null=True,
+        blank=True, null=True, db_index=True,
         verbose_name=_("Valid until")
     )
     block_quota = models.BooleanField(