Add partial index on projects for index-only scans.

This helps with queries that get project ids based on the - comparably
rare - visibility levels 10 and 20. For these, postgres can now leverage
the partial index for a index-only scan to improve performance.

Example queries:
SELECT id FROM projects WHERE visibility_level IN (10,20)
SELECT id FROM projects WHERE visibility_level IN (10)

For MySQL, this results in a full index on id because MySQL omits the
WHERE clause. That is, the index is a duplicate of the primary key
basically.

db/migrate/20180213131630_add_partial_index_to_projects_for_index_only_scans.rb

+class AddPartialIndexToProjectsForIndexOnlyScans < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+  INDEX_NAME = 'index_projects_on_id_partial_for_visibility'
+
+  disable_ddl_transaction!
+
+  # Adds a partial index to leverage index-only scans when looking up project ids
+  def up
+    unless index_exists?(:projects, :id, name: INDEX_NAME)
+      add_concurrent_index :projects, :id, name: INDEX_NAME, unique: true, where: 'visibility_level IN (10,20)'
+    end
+  end
+
+  def down
+    if index_exists?(:projects, :id, name: INDEX_NAME)
+      remove_concurrent_index :projects, :id, name: INDEX_NAME, unique: true, where: 'visibility_level IN (10,20)'
+    end
+  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: 20180208183958) do
+ActiveRecord::Schema.define(version: 20180213131630) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -1468,6 +1468,7 @@ ActiveRecord::Schema.define(version: 20180208183958) do
   add_index "projects", ["created_at"], name: "index_projects_on_created_at", using: :btree
   add_index "projects", ["creator_id"], name: "index_projects_on_creator_id", using: :btree
   add_index "projects", ["description"], name: "index_projects_on_description_trigram", using: :gin, opclasses: {"description"=>"gin_trgm_ops"}
+  add_index "projects", ["id"], name: "index_projects_on_id_partial_for_visibility", unique: true, where: "(visibility_level = ANY (ARRAY[10, 20]))", using: :btree
   add_index "projects", ["last_activity_at"], name: "index_projects_on_last_activity_at", using: :btree
   add_index "projects", ["last_repository_check_failed"], name: "index_projects_on_last_repository_check_failed", using: :btree
   add_index "projects", ["last_repository_updated_at"], name: "index_projects_on_last_repository_updated_at", using: :btree