Enhance performance of counting local LFS objects

Add an index to the `file_store` column on `lfs_objects`. This makes
counting local objects faster.

Also, there is no longer need to check for objects with `file_store`
being `NULL`. See
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/18557

---

### Query plans

#### Before & with `NULL`

```
Aggregate  (cost=113495.96..113495.97 rows=1 width=8) (actual time=1691.394..1691.394 rows=1 loops=1)
  ->  Seq Scan on lfs_objects  (cost=0.00..106415.50 rows=2832186 width=0) (actual time=0.012..1312.488 rows=2852607 loops=1)
        Filter: ((file_store = 1) OR (file_store IS NULL))
        Rows Removed by Filter: 131
Planning time: 0.077 ms
Execution time: 1691.433 ms
```

#### Before, without `NULL`

```
Aggregate  (cost=113495.96..113495.97 rows=1 width=8) (actual time=856.423..856.424 rows=1 loops=1)
  ->  Seq Scan on lfs_objects  (cost=0.00..106415.50 rows=2832186 width=0) (actual time=0.012..672.181 rows=2852607 loops=1)
        Filter: (file_store = 1)
        Rows Removed by Filter: 131
Planning time: 0.128 ms
Execution time: 856.470 ms
```

#### After & with `NULL`

```
Aggregate  (cost=68819.95..68819.96 rows=1 width=8) (actual time=583.355..583.355 rows=1 loops=1)
  ->  Index Only Scan using index_lfs_objects_on_file_store on lfs_objects  (cost=0.43..61688.35 rows=2852643 width=0) (actual time=0.028..399.177 rows=2852607 loops=1)
        Filter: ((file_store = 1) OR (file_store IS NULL))
        Rows Removed by Filter: 131
        Heap Fetches: 867
Planning time: 0.096 ms
Execution time: 583.404 ms
```

#### After, without `NULL`

```
Aggregate  (cost=68817.29..68817.30 rows=1 width=8) (actual time=490.550..490.551 rows=1 loops=1)
  ->  Index Only Scan using index_lfs_objects_on_file_store on lfs_objects  (cost=0.43..61685.68 rows=2852643 width=0) (actual time=0.040..311.760 rows=2852607 loops=1)
        Index Cond: (file_store = 1)
        Heap Fetches: 831
Planning time: 0.294 ms
Execution time: 490.590 ms
```

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/6067

1 files changed, 2 insertions, 2 deletions

diff --git a/app/models/lfs_object.rb b/app/models/lfs_object.rb
index 97bf5d611c2..69c563545bb 100644
--- a/app/models/lfs_object.rb
+++ b/app/models/lfs_object.rb
@@ -7,7 +7,7 @@ class LfsObject < ActiveRecord::Base
   has_many :lfs_objects_projects, dependent: :destroy # rubocop:disable Cop/ActiveRecordDependent
   has_many :projects, through: :lfs_objects_projects
 
-  scope :with_files_stored_locally, -> { where(file_store: [nil, LfsObjectUploader::Store::LOCAL]) }
+  scope :with_files_stored_locally, -> { where(file_store: LfsObjectUploader::Store::LOCAL) }
 
   validates :oid, presence: true, uniqueness: true
 
@@ -26,7 +26,7 @@ class LfsObject < ActiveRecord::Base
   end
 
   def local_store?
-    [nil, LfsObjectUploader::Store::LOCAL].include?(self.file_store)
+    file_store == LfsObjectUploader::Store::LOCAL
   end
 
   # rubocop: disable DestroyAll