Add (partial) index on Labels.template

changelogs/unreleased/add-labels-template-index.yml

+---
+title: Add (partial) index on Labels.template
+merge_request:
+author:
+type: other

db/migrate/20170927122209_add_partial_index_for_labels_template.rb

+# See http://doc.gitlab.com/ce/development/migration_style_guide.html
+# for more information on how to write migrations for GitLab.
+
+class AddPartialIndexForLabelsTemplate < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  # Set this constant to true if this migration requires downtime.
+  DOWNTIME = false
+
+  # When a migration requires downtime you **must** uncomment the following
+  # constant and define a short and easy to understand explanation as to why the
+  # migration requires downtime.
+  # DOWNTIME_REASON = ''
+
+  # When using the methods "add_concurrent_index", "remove_concurrent_index" or
+  # "add_column_with_default" you must disable the use of transactions
+  # as these methods can not run in an existing transaction.
+  # When using "add_concurrent_index" or "remove_concurrent_index" methods make sure
+  # that either of them is the _only_ method called in the migration,
+  # any other changes should go in a separate migration.
+  # This ensures that upon failure _only_ the index creation or removing fails
+  # and can be retried or reverted easily.
+  #
+  # To disable transactions uncomment the following line and remove these
+  # comments:
+
+  disable_ddl_transaction!
+
+  # Note this is a partial index in Postgres but MySQL will ignore the
+  # partial index clause. By making it an index on "template" this
+  # means the index will still accomplish the same goal of optimizing
+  # a query with "where template = true" on MySQL -- it'll just take
+  # more space. In this case the number of records with template=true
+  # is expected to be very small (small enough to display on a single
+  # web page) so it's ok to filter or sort them without the index
+  # anyways.
+
+  def up
+    add_concurrent_index "labels", ["template"], where: "template"
+  end
+
+  def down
+    remove_concurrent_index "labels", ["template"], where: "template"
+  end
+end

db/schema.rb

@@ -730,6 +730,7 @@ ActiveRecord::Schema.define(version: 20170928100231) do
   add_index "labels", ["group_id", "project_id", "title"], name: "index_labels_on_group_id_and_project_id_and_title", unique: true, using: :btree
   add_index "labels", ["project_id"], name: "index_labels_on_project_id", using: :btree
+  add_index "labels", ["template"], name: "index_labels_on_template", where: "template", using: :btree
   add_index "labels", ["title"], name: "index_labels_on_title", using: :btree
   add_index "labels", ["type", "project_id"], name: "index_labels_on_type_and_project_id", using: :btree