Track index bloat estimate

This allows to check the quality of the estimate after the fact: The
difference in index size (after reindexing) should match the bloat
estimate.

changelogs/unreleased/ab-track-bloat.yml

+---
+title: Track index bloat estimate
+merge_request: 49822
+author:
+type: other

db/migrate/20201211145950_add_bloat_estimate_to_reindex_action.rb

+# frozen_string_literal: true
+
+class AddBloatEstimateToReindexAction < ActiveRecord::Migration[6.0]
+  DOWNTIME = false
+
+  def change
+    add_column :postgres_reindex_actions, :bloat_estimate_bytes_start, :bigint
+  end
+end

db/schema_migrations/20201211145950

+ecf6b392f35bb0ef905144a4605bcb927ce767240e47ec3b0653a94139b987bd
\ No newline at end of file

db/structure.sql

@@ -15216,6 +15216,7 @@ CREATE TABLE postgres_reindex_actions (
     ondisk_size_bytes_end bigint,
     state smallint DEFAULT 0 NOT NULL,
     index_identifier text NOT NULL,
+    bloat_estimate_bytes_start bigint,
     CONSTRAINT check_f12527622c CHECK ((char_length(index_identifier) <= 255))
 );
 

lib/gitlab/database/reindexing/reindex_action.rb

@@ -18,7 +18,8 @@ module Gitlab
           action = create!(
             index_identifier: index.identifier,
             action_start: Time.zone.now,
-            ondisk_size_bytes_start: index.ondisk_size_bytes
+            ondisk_size_bytes_start: index.ondisk_size_bytes,
+            bloat_estimate_bytes_start: index.bloat_size
           )
 
           yield

spec/lib/gitlab/database/reindexing/reindex_action_spec.rb

@@ -3,7 +3,7 @@
 require 'spec_helper'
 
 RSpec.describe Gitlab::Database::Reindexing::ReindexAction, '.keep_track_of' do
-  let(:index) { double('index', identifier: 'public.something', ondisk_size_bytes: 10240, reload: nil) }
+  let(:index) { double('index', identifier: 'public.something', ondisk_size_bytes: 10240, reload: nil, bloat_size: 42) }
   let(:size_after) { 512 }
 
   it 'yields to the caller' do
@@ -47,6 +47,12 @@ RSpec.describe Gitlab::Database::Reindexing::ReindexAction, '.keep_track_of' do
     expect(find_record.ondisk_size_bytes_end).to eq(size_after)
   end
 
+  it 'creates the record with the indexes bloat estimate' do
+    described_class.keep_track_of(index) do
+      expect(find_record.bloat_estimate_bytes_start).to eq(index.bloat_size)
+    end
+  end
+
   context 'in case of errors' do
     it 'sets the state to failed' do
       expect do