diff --git a/manila/db/api.py b/manila/db/api.py
index ccdb5c88b5..c37b80a856 100644
--- a/manila/db/api.py
+++ b/manila/db/api.py
@@ -386,6 +386,11 @@ def share_instance_get_all_by_share_group_id(context, share_group_id):
     return IMPL.share_instance_get_all_by_share_group_id(
         context, share_group_id)
 
+
+def share_instance_sizes_sum_by_host(context, host):
+    """Returns sum of sizes of all share instances on given host."""
+    return IMPL.share_instance_sizes_sum_by_host(context, host)
+
 ###################
 
 
diff --git a/manila/db/sqlalchemy/api.py b/manila/db/sqlalchemy/api.py
index 6f97d9e416..0790b62626 100644
--- a/manila/db/sqlalchemy/api.py
+++ b/manila/db/sqlalchemy/api.py
@@ -1997,7 +1997,6 @@ def share_instance_get_all_by_host(context, host, with_share_data=False,
     )
     if status is not None:
         instances = instances.filter(models.ShareInstance.status == status)
-
     if with_share_data:
         instances = _set_instances_share_data(instances)
     else:
@@ -2006,6 +2005,20 @@ def share_instance_get_all_by_host(context, host, with_share_data=False,
     return instances
 
 
+@require_context
+@context_manager.reader
+def share_instance_sizes_sum_by_host(context, host):
+    result = model_query(
+        context, models.Share, func.sum(models.Share.size),
+    ).join(
+        models.ShareInstance.share,
+    ).filter(or_(
+        models.ShareInstance.host == host,
+        models.ShareInstance.host.like("{0}#%".format(host)),
+    )).first()
+    return int(result[0] or 0)
+
+
 @require_context
 @context_manager.reader
 def share_instance_get_all_by_share_network(context, share_network_id):
diff --git a/manila/scheduler/host_manager.py b/manila/scheduler/host_manager.py
index 60570bc06b..1663254c19 100644
--- a/manila/scheduler/host_manager.py
+++ b/manila/scheduler/host_manager.py
@@ -451,15 +451,7 @@ class PoolState(HostState):
 
     def _estimate_provisioned_capacity(self, host_name, context=None):
         """Estimate provisioned capacity from share sizes on backend."""
-        provisioned_capacity = 0
-
-        instances = db.share_instance_get_all_by_host(
-            context, host_name, with_share_data=True)
-
-        for instance in instances:
-            # Size of share instance that's still being created, will be None.
-            provisioned_capacity += instance['size'] or 0
-        return provisioned_capacity
+        return db.share_instance_sizes_sum_by_host(context, host_name)
 
     @utils.synchronized("update_from_share_capability")
     def update_from_share_capability(
diff --git a/manila/tests/db/sqlalchemy/test_api.py b/manila/tests/db/sqlalchemy/test_api.py
index 2c3b654f52..950ddec4a4 100644
--- a/manila/tests/db/sqlalchemy/test_api.py
+++ b/manila/tests/db/sqlalchemy/test_api.py
@@ -602,6 +602,18 @@ class ShareDatabaseAPITestCase(test.TestCase):
 
         self.assertEqual(0, len(instances))
 
+    @ddt.data(
+        {'status': constants.STATUS_AVAILABLE},
+        {'status': None})
+    @ddt.unpack
+    def test_share_instance_get_all_by_host_no_instance(self, status):
+        db_utils.create_share_without_instance()
+        instances = db_api.share_instance_get_all_by_host(
+            self.ctxt, "fake_host", with_share_data=True, status=status
+        )
+
+        self.assertEqual(0, len(instances))
+
     def test_share_instance_get_all_by_share_group(self):
         group = db_utils.create_share_group()
         db_utils.create_share(share_group_id=group['id'])
diff --git a/manila/tests/scheduler/test_host_manager.py b/manila/tests/scheduler/test_host_manager.py
index ef5904f705..ea04f0704c 100644
--- a/manila/tests/scheduler/test_host_manager.py
+++ b/manila/tests/scheduler/test_host_manager.py
@@ -1251,9 +1251,10 @@ class PoolStateTestCase(test.TestCase):
     @ddt.unpack
     def test_update_from_share_capability(self, share_capability, instances):
         fake_context = context.RequestContext('user', 'project', is_admin=True)
+        sizes = [instance['size'] or 0 for instance in instances]
         self.mock_object(
-            db, 'share_instance_get_all_by_host',
-            mock.Mock(return_value=instances))
+            db, 'share_instance_sizes_sum_by_host',
+            mock.Mock(return_value=sum(sizes)))
         fake_pool = host_manager.PoolState('host1', None, 'pool0')
         self.assertIsNone(fake_pool.free_capacity_gb)
 
@@ -1276,19 +1277,19 @@ class PoolStateTestCase(test.TestCase):
             self.assertEqual(thin_provisioned, fake_pool.thin_provisioning)
             if 'provisioned_capacity_gb' not in share_capability or (
                     share_capability['provisioned_capacity_gb'] is None):
-                db.share_instance_get_all_by_host.assert_called_once_with(
-                    fake_context, fake_pool.host, with_share_data=True)
+                db.share_instance_sizes_sum_by_host.assert_called_once_with(
+                    fake_context, fake_pool.host)
                 if len(instances) > 0:
                     self.assertEqual(4, fake_pool.provisioned_capacity_gb)
                 else:
                     self.assertEqual(0, fake_pool.provisioned_capacity_gb)
             else:
-                self.assertFalse(db.share_instance_get_all_by_host.called)
+                self.assertFalse(db.share_instance_sizes_sum_by_host.called)
                 self.assertEqual(share_capability['provisioned_capacity_gb'],
                                  fake_pool.provisioned_capacity_gb)
         else:
             self.assertFalse(fake_pool.thin_provisioning)
-            self.assertFalse(db.share_instance_get_all_by_host.called)
+            self.assertFalse(db.share_instance_sizes_sum_by_host.called)
             if 'provisioned_capacity_gb' not in share_capability or (
                     share_capability['provisioned_capacity_gb'] is None):
                 self.assertIsNone(fake_pool.provisioned_capacity_gb)
diff --git a/releasenotes/notes/bug-2020187-scheduler-performance-2edc4c706b2fea2f.yaml b/releasenotes/notes/bug-2020187-scheduler-performance-2edc4c706b2fea2f.yaml
new file mode 100644
index 0000000000..4dca98b468
--- /dev/null
+++ b/releasenotes/notes/bug-2020187-scheduler-performance-2edc4c706b2fea2f.yaml
@@ -0,0 +1,5 @@
+---
+fixes:
+  - |
+    Improve scheduler performance to estimate the allocated capacity for thin
+    provisioning hosts.