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

2 files changed, 18 insertions, 0 deletions

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|