Add confidential notes index

Adds an index on confidential notes using ID field and removes
old index which used confidential field (which was not ideal for
iterating through confidential notes in batches).

Changelog: added

db/post_migrate/20220613095911_create_confidential_notes_index_on_id.rb

+# frozen_string_literal: true
+
+class CreateConfidentialNotesIndexOnId < Gitlab::Database::Migration[2.0]
+  OLD_INDEX_NAME = 'index_notes_on_confidential'
+  INDEX_NAME = 'index_notes_on_id_where_confidential'
+
+  disable_ddl_transaction!
+
+  def up
+    remove_concurrent_index_by_name :notes, name: OLD_INDEX_NAME
+    add_concurrent_index :notes, :id, where: 'confidential = true', name: INDEX_NAME
+  end
+
+  def down
+    # we don't have to re-create OLD_INDEX_NAME index
+    # because it wasn't used yet, also its creation might be expensive
+    remove_concurrent_index_by_name :notes, name: INDEX_NAME
+  end
+end

db/schema_migrations/20220613095911

+1d2dc45d6fae911d75eaf5970afbae6d2f31d2efd1c27b75fce5feacbcc319d3
\ No newline at end of file

db/structure.sql

@@ -28614,12 +28614,12 @@ CREATE INDEX index_notes_on_author_id_and_created_at_and_id ON notes USING btree
 CREATE INDEX index_notes_on_commit_id ON notes USING btree (commit_id);
-CREATE INDEX index_notes_on_confidential ON notes USING btree (confidential) WHERE (confidential = true);
-
 CREATE INDEX index_notes_on_created_at ON notes USING btree (created_at);
 CREATE INDEX index_notes_on_discussion_id ON notes USING btree (discussion_id);
+CREATE INDEX index_notes_on_id_where_confidential ON notes USING btree (id) WHERE (confidential = true);
+
 CREATE INDEX index_notes_on_line_code ON notes USING btree (line_code);
 CREATE INDEX index_notes_on_noteable_id_and_noteable_type_and_system ON notes USING btree (noteable_id, noteable_type, system);