Enhance performance of counting local Uploads

Add an index to the `store` column on `uploads`. This makes counting
local uploads faster.

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

---

### Query plans

Query:

```sql
SELECT COUNT(*)
FROM "uploads"
WHERE ("uploads"."store" = ? OR "uploads"."store" IS NULL)
```

#### Without index

```
gitlabhq_production=# EXPLAIN ANALYZE SELECT uploads.* FROM uploads WHERE (uploads.store = 1 OR uploads.store IS NULL);
                                                  QUERY PLAN
---------------------------------------------------------------------------------------------------------------
 Seq Scan on uploads  (cost=0.00..601729.54 rows=578 width=272) (actual time=6.170..2308.256 rows=545 loops=1)
   Filter: ((store = 1) OR (store IS NULL))
   Rows Removed by Filter: 4411957
 Planning time: 38.652 ms
 Execution time: 2308.454 ms
(5 rows)
```

#### Add index

```
gitlabhq_production=# create index uploads_tmp1 on uploads (store);
CREATE INDEX
```

#### With index

```
gitlabhq_production=# EXPLAIN ANALYZE SELECT uploads.* FROM uploads WHERE (uploads.store = 1 OR uploads.store IS NULL);
                                                          QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------------
 Bitmap Heap Scan on uploads  (cost=11.46..1238.88 rows=574 width=272) (actual time=0.155..0.577 rows=545 loops=1)
   Recheck Cond: ((store = 1) OR (store IS NULL))
   Heap Blocks: exact=217
   ->  BitmapOr  (cost=11.46..11.46 rows=574 width=0) (actual time=0.116..0.116 rows=0 loops=1)
         ->  Bitmap Index Scan on uploads_tmp1  (cost=0.00..8.74 rows=574 width=0) (actual time=0.095..0.095 rows=545 loops=1)
               Index Cond: (store = 1)
         ->  Bitmap Index Scan on uploads_tmp1  (cost=0.00..2.44 rows=1 width=0) (actual time=0.020..0.020 rows=0 loops=1)
               Index Cond: (store IS NULL)
 Planning time: 0.274 ms
 Execution time: 0.637 ms
(10 rows)
```

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/6070

6 files changed, 34 insertions, 18 deletions

diff --git a/app/models/upload.rb b/app/models/upload.rb
index 23bc9ca42fc..a6af7a8b869 100644
--- a/app/models/upload.rb
+++ b/app/models/upload.rb
@@ -11,7 +11,7 @@ class Upload < ActiveRecord::Base
   validates :model, presence: true
   validates :uploader, presence: true
 
-  scope :with_files_stored_locally, -> { where(store: [nil, ObjectStorage::Store::LOCAL]) }
+  scope :with_files_stored_locally, -> { where(store: ObjectStorage::Store::LOCAL) }
 
   before_save  :calculate_checksum!, if: :foreground_checksummable?
   after_commit :schedule_checksum,   if: :checksummable?
@@ -57,8 +57,6 @@ class Upload < ActiveRecord::Base
   end
 
   def local?
-    return true if store.nil?
-
     store == ObjectStorage::Store::LOCAL
   end
 
diff --git a/changelogs/unreleased/tc-index-uploads-file-store.yml b/changelogs/unreleased/tc-index-uploads-file-store.yml
new file mode 100644
index 00000000000..fa3b3164e38
--- /dev/null
+++ b/changelogs/unreleased/tc-index-uploads-file-store.yml
@@ -0,0 +1,5 @@
+---
+title: Enhance performance of counting local Uploads
+merge_request: 22522
+author:
+type: performance
diff --git a/db/migrate/20181005125926_add_index_to_uploads_store.rb b/db/migrate/20181005125926_add_index_to_uploads_store.rb
new file mode 100644
index 00000000000..d32ca05e980
--- /dev/null
+++ b/db/migrate/20181005125926_add_index_to_uploads_store.rb
@@ -0,0 +1,17 @@
+# frozen_string_literal: true
+
+class AddIndexToUploadsStore < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index :uploads, :store
+  end
+
+  def down
+    remove_concurrent_index :uploads, :store
+  end
+end
diff --git a/db/schema.rb b/db/schema.rb
index cfbfd7ad375..7509941325f 100644
--- a/db/schema.rb
+++ b/db/schema.rb
@@ -2155,6 +2155,7 @@ ActiveRecord::Schema.define(version: 20181107054254) do
 
   add_index "uploads", ["checksum"], name: "index_uploads_on_checksum", using: :btree
   add_index "uploads", ["model_id", "model_type"], name: "index_uploads_on_model_id_and_model_type", using: :btree
+  add_index "uploads", ["store"], name: "index_uploads_on_store", using: :btree
   add_index "uploads", ["uploader", "path"], name: "index_uploads_on_uploader_and_path", using: :btree
 
   create_table "user_agent_details", force: :cascade do |t|
diff --git a/spec/models/upload_spec.rb b/spec/models/upload_spec.rb
index 3c89e99abf0..e3544b84b50 100644
--- a/spec/models/upload_spec.rb
+++ b/spec/models/upload_spec.rb
@@ -21,7 +21,8 @@ describe Upload do
           path: __FILE__,
           size: described_class::CHECKSUM_THRESHOLD + 1.kilobyte,
           model: build_stubbed(:user),
-          uploader: double('ExampleUploader')
+          uploader: double('ExampleUploader'),
+          store: ObjectStorage::Store::LOCAL
         )
 
         expect(UploadChecksumWorker)
@@ -35,7 +36,8 @@ describe Upload do
           path: __FILE__,
           size: described_class::CHECKSUM_THRESHOLD,
           model: build_stubbed(:user),
-          uploader: double('ExampleUploader')
+          uploader: double('ExampleUploader'),
+          store: ObjectStorage::Store::LOCAL
         )
 
         expect { upload.save }
@@ -60,7 +62,7 @@ describe Upload do
   describe '#absolute_path' do
     it 'returns the path directly when already absolute' do
       path = '/path/to/namespace/project/secret/file.jpg'
-      upload = described_class.new(path: path)
+      upload = described_class.new(path: path, store: ObjectStorage::Store::LOCAL)
 
       expect(upload).not_to receive(:uploader_class)
 
@@ -69,7 +71,7 @@ describe Upload do
 
     it "delegates to the uploader's absolute_path method" do
       uploader = spy('FakeUploader')
-      upload = described_class.new(path: 'secret/file.jpg')
+      upload = described_class.new(path: 'secret/file.jpg', store: ObjectStorage::Store::LOCAL)
       expect(upload).to receive(:uploader_class).and_return(uploader)
 
       upload.absolute_path
@@ -81,7 +83,8 @@ describe Upload do
   describe '#calculate_checksum!' do
     let(:upload) do
       described_class.new(path: __FILE__,
-                          size: described_class::CHECKSUM_THRESHOLD - 1.megabyte)
+                          size: described_class::CHECKSUM_THRESHOLD - 1.megabyte,
+                          store: ObjectStorage::Store::LOCAL)
     end
 
     it 'sets `checksum` to SHA256 sum of the file' do
@@ -104,13 +107,13 @@ describe Upload do
 
   describe '#exist?' do
     it 'returns true when the file exists' do
-      upload = described_class.new(path: __FILE__)
+      upload = described_class.new(path: __FILE__, store: ObjectStorage::Store::LOCAL)
 
       expect(upload).to exist
     end
 
     it 'returns false when the file does not exist' do
-      upload = described_class.new(path: "#{__FILE__}-nope")
+      upload = described_class.new(path: "#{__FILE__}-nope", store: ObjectStorage::Store::LOCAL)
 
       expect(upload).not_to exist
     end
diff --git a/spec/tasks/gitlab/uploads/migrate_rake_spec.rb b/spec/tasks/gitlab/uploads/migrate_rake_spec.rb
index 6fcfae358ec..9588e8be5dc 100644
--- a/spec/tasks/gitlab/uploads/migrate_rake_spec.rb
+++ b/spec/tasks/gitlab/uploads/migrate_rake_spec.rb
@@ -38,14 +38,6 @@ describe 'gitlab:uploads:migrate rake tasks' do
       let!(:projects) { create_list(:project, 10, :with_avatar) }
 
       it_behaves_like 'enqueue jobs in batch', batch: 4
-
-      context 'Upload has store = nil' do
-        before do
-          Upload.where(model: projects).update_all(store: nil)
-        end
-
-        it_behaves_like 'enqueue jobs in batch', batch: 4
-      end
     end
 
     context "for Group" do