Create a single temporary index for note mentions migration

Replace separate temporary indexes for each noteable_type with
a single index for all noteable_types.

changelogs/unreleased/create_single_temporary_index_for_notes_mentions.yml

+---
+title: Replace several temporary indexes with a single one to save time when running mentions migration
+merge_request:
+author:
+type: performance

db/post_migrate/20200128132510_add_temporary_index_for_notes_with_mentions.rb

+# frozen_string_literal: true
+
+class AddTemporaryIndexForNotesWithMentions < ActiveRecord::Migration[5.2]
+  include Gitlab::Database::MigrationHelpers
+
+  disable_ddl_transaction!
+
+  INDEX_CONDITION = "note LIKE '%@%'::text"
+  INDEX_NAME = 'note_mentions_temp_index'
+
+  EPIC_MENTIONS_INDEX_NAME = 'epic_mentions_temp_index'
+  DESIGN_MENTIONS_INDEX_NAME = 'design_mentions_temp_index'
+
+  def up
+    # create temporary index for notes with mentions, may take well over 1h
+    add_concurrent_index(:notes, [:id, :noteable_type], where: INDEX_CONDITION, name: INDEX_NAME)
+
+    # cleanup previous temporary indexes, as we'll be usig the single one
+    remove_concurrent_index(:notes, :id, name: EPIC_MENTIONS_INDEX_NAME)
+    remove_concurrent_index(:notes, :id, name: DESIGN_MENTIONS_INDEX_NAME)
+  end
+
+  def down
+    remove_concurrent_index(:notes, :id, name: INDEX_NAME)
+
+    add_concurrent_index(:notes, :id, where: "#{INDEX_CONDITION} AND noteable_type='Epic'", name: EPIC_MENTIONS_INDEX_NAME)
+    add_concurrent_index(:notes, :id, where: "#{INDEX_CONDITION} AND noteable_type='DesignManagement::Design'", name: DESIGN_MENTIONS_INDEX_NAME)
+  end
+end

db/schema.rb

@@ -2840,8 +2840,7 @@ ActiveRecord::Schema.define(version: 2020_03_10_135823) do
     t.index ["commit_id"], name: "index_notes_on_commit_id"
     t.index ["created_at"], name: "index_notes_on_created_at"
     t.index ["discussion_id"], name: "index_notes_on_discussion_id"
-    t.index ["id"], name: "design_mentions_temp_index", where: "((note ~~ '%@%'::text) AND ((noteable_type)::text = 'DesignManagement::Design'::text))"
-    t.index ["id"], name: "epic_mentions_temp_index", where: "((note ~~ '%@%'::text) AND ((noteable_type)::text = 'Epic'::text))"
+    t.index ["id", "noteable_type"], name: "note_mentions_temp_index", where: "(note ~~ '%@%'::text)"
     t.index ["line_code"], name: "index_notes_on_line_code"
     t.index ["note"], name: "index_notes_on_note_trigram", opclass: :gin_trgm_ops, using: :gin
     t.index ["note"], name: "tmp_idx_on_promoted_notes", where: "(((noteable_type)::text = 'Issue'::text) AND (system IS TRUE) AND (note ~~ 'promoted to epic%'::text))"