Add duplicated gin trigram index on notes note

We identified that the gin trigram index on notes (note)
can lead to very slow insert times on notes. It appears that
reindexing the index solves the problem. We have decided to
reindex it through migrations, first by creating a duplicate
index, then by dropping the old index.

Original MR thread: gitlab.com/gitlab-org/gitlab/-/issues/218410#note_565624409
Change MR: gitlab.com/gitlab-org/gitlab/-/merge_requests/61430

changelogs/unreleased/add_duplicate_notes_note_trigram_index.yml

+---
+title: Add duplicated gin trigram index on notes table (note) to replace existing
+merge_request: 61430
+author:
+type: performance

db/migrate/20210511051718_create_index_on_notes_note.rb

+# frozen_string_literal: true
+
+# See https://docs.gitlab.com/ee/development/migration_style_guide.html
+# for more information on how to write migrations for GitLab.
+
+class CreateIndexOnNotesNote < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DUPLICATE_INDEX_NAME = 'index_notes_on_note_gin_trigram'
+  CURRENT_INDEX_NAME = 'index_notes_on_note_trigram'
+
+  disable_ddl_transaction!
+
+  # https://gitlab.com/gitlab-org/gitlab/-/issues/218410#note_565624409
+  # We are having troubles with the index, and some inserts are taking a long time
+  # so in this migration we are recreating the index
+  def up
+    add_concurrent_index :notes, :note, name: DUPLICATE_INDEX_NAME, using: :gin, opclass: :gin_trgm_ops
+    remove_concurrent_index_by_name :notes, CURRENT_INDEX_NAME
+  end
+
+  def down
+    remove_concurrent_index_by_name :notes, DUPLICATE_INDEX_NAME
+    add_concurrent_index :notes, :note, name: CURRENT_INDEX_NAME, using: :gin, opclass: :gin_trgm_ops
+  end
+end

db/schema_migrations/20210511051718

+2d11da499f49964f37cc0a2c541cf58182416ae7b6b0e762a135b327099de1a4
\ No newline at end of file

db/structure.sql

@@ -23667,7 +23667,7 @@ CREATE INDEX index_notes_on_discussion_id ON notes USING btree (discussion_id);
 
 CREATE INDEX index_notes_on_line_code ON notes USING btree (line_code);
 
-CREATE INDEX index_notes_on_note_trigram ON notes USING gin (note gin_trgm_ops);
+CREATE INDEX index_notes_on_note_gin_trigram ON notes USING gin (note gin_trgm_ops);
 
 CREATE INDEX index_notes_on_noteable_id_and_noteable_type_and_system ON notes USING btree (noteable_id, noteable_type, system);