Added partial index for merge requests

This index is added on `(target_project_id, iid)` and has a `WHERE state
= 'opened'` condition. Using this index we can drastically improve the
performance of the query used to count the total number of merge
requests in a group. Without this index the query would eventually
perform the following:

    ->  Index Scan using index_merge_requests_on_target_project_id_and_iid on merge_requests  (cost=0.43..4.89 rows=7 width=4) (actual time=0.058..0.353 rows=6 loops=228)
          Index Cond: (target_project_id = projects.id)
          Filter: ((state)::text = 'opened'::text)
          Rows Removed by Filter: 141
          Buffers: shared hit=34351 dirtied=1

Out of the ~180 milliseconds the entire query would take, around 170
milliseconds was spent in just this segment. With the index in place,
the above segment is turned into the following:

    ->  Index Only Scan using yorick_test on merge_requests  (cost=0.42..0.55 rows=7 width=4) (actual time=0.004..0.010 rows=6 loops=228)
          Index Cond: (target_project_id = projects.id)
          Heap Fetches: 419
          Buffers: shared hit=1381

The index also reduces the total query time to roughly 10 milliseconds.

db/migrate/20180524132016_merge_requests_target_id_iid_state_partial_index.rb

+# See http://doc.gitlab.com/ce/development/migration_style_guide.html
+# for more information on how to write migrations for GitLab.
+
+class MergeRequestsTargetIdIidStatePartialIndex < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  # Set this constant to true if this migration requires downtime.
+  DOWNTIME = false
+
+  INDEX_NAME = 'index_merge_requests_on_target_project_id_and_iid_opened'
+
+  disable_ddl_transaction!
+
+  def up
+    # On GitLab.com this index will take up roughly 5 MB of space.
+    add_concurrent_index(
+      :merge_requests,
+      [:target_project_id, :iid],
+      where: "state = 'opened'",
+      name: INDEX_NAME
+    )
+  end
+
+  def down
+    remove_concurrent_index_by_name(:merge_requests, INDEX_NAME)
+  end
+end

db/schema.rb

@@ -11,7 +11,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20180521171529) do
+ActiveRecord::Schema.define(version: 20180524132016) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -1232,6 +1232,7 @@ ActiveRecord::Schema.define(version: 20180521171529) do
   add_index "merge_requests", ["source_project_id", "source_branch"], name: "index_merge_requests_on_source_project_id_and_source_branch", using: :btree
   add_index "merge_requests", ["target_branch"], name: "index_merge_requests_on_target_branch", using: :btree
   add_index "merge_requests", ["target_project_id", "iid"], name: "index_merge_requests_on_target_project_id_and_iid", unique: true, using: :btree
+  add_index "merge_requests", ["target_project_id", "iid"], name: "index_merge_requests_on_target_project_id_and_iid_opened", where: "((state)::text = 'opened'::text)", using: :btree
   add_index "merge_requests", ["target_project_id", "merge_commit_sha", "id"], name: "index_merge_requests_on_tp_id_and_merge_commit_sha_and_id", using: :btree
   add_index "merge_requests", ["title"], name: "index_merge_requests_on_title", using: :btree
   add_index "merge_requests", ["title"], name: "index_merge_requests_on_title_trigram", using: :gin, opclasses: {"title"=>"gin_trgm_ops"}