Remove projects index on mirror and mirror_last_update_at

ee/changelogs/unreleased/53991-remove-partial-index-for-projects.yml

+---
+title: Remove partial index for projects on mirror and mirror_last_update_at
+merge_request: 8585
+author:
+type: performance

ee/db/migrate/20181126125616_remove_projects_index_on_mirror_and_mirror_at.rb

+# frozen_string_literal: true
+
+# See http://doc.gitlab.com/ce/development/migration_style_guide.html
+# for more information on how to write migrations for GitLab.
+
+class RemoveProjectsIndexOnMirrorAndMirrorAt < ActiveRecord::Migration[5.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    remove_concurrent_index(*index_arguments)
+  end
+
+  def down
+    add_concurrent_index(*index_arguments)
+  end
+
+  private
+
+  def index_arguments
+    [
+      :projects,
+      [:mirror, :mirror_last_update_at],
+      {
+        name: 'index_projects_on_mirror_and_mirror_last_update_at',
+        where: 'mirror'
+      }
+    ]
+  end
+end