Remove trigram index on notes

This removes the GIN index on notes because these are expensive to
update. This causes note creation to sometimes time out.

This query is also seldom used. We only search for notes in the Global
Search page and only searches within projects are allowed. And when
searching within a project, this global index is unlikey to be used and
might even degrade performance as we see in issue searches. For issue
searches within a project, we use a CTE to avoid the global trigram
index.

Changelog: performance

db/migrate/20211008043855_remove_notes_trigram_index.rb

+# frozen_string_literal: true
+
+class RemoveNotesTrigramIndex < Gitlab::Database::Migration[1.0]
+  disable_ddl_transaction!
+
+  NOTES_TRIGRAM_INDEX_NAME = 'index_notes_on_note_trigram'
+
+  def up
+    remove_concurrent_index_by_name(:notes, NOTES_TRIGRAM_INDEX_NAME)
+  end
+
+  def down
+    add_concurrent_index :notes, :note, name: NOTES_TRIGRAM_INDEX_NAME, using: :gin, opclass: { content: :gin_trgm_ops }
+  end
+end

db/schema_migrations/20211008043855

+166ae24ae4856488c81a71c650dca038c8cd7cb2221545e84431e118da097688
\ No newline at end of file

db/structure.sql

@@ -26539,8 +26539,6 @@ 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_noteable_id_and_noteable_type_and_system ON notes USING btree (noteable_id, noteable_type, system);
 
 CREATE INDEX index_notes_on_project_id_and_id_and_system_false ON notes USING btree (project_id, id) WHERE (NOT system);