Expand index selection strategy for reindexing

We change strategy to select indexes for reindexing:

1. Consider relative bloat levels instead of absolute bloat
1. Exclude GIN indexes (risk for GitLab.com)
1. Exclude indexes with relative bloat level < 20%
1. Exclude indexes < 1 GB ondisk size (not worth it for now)
1. Exclude indexes > 100 GB ondisk size (risk for GitLab.com)
1. Don't reindex an index twice within 10 days

Together with adding support for unique indexex in
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/64695, this
aims to distribute reindexing actions better across indexes and exclude
the risky ones.

Relates to:

- https://gitlab.com/gitlab-org/gitlab/-/issues/335295
- https://gitlab.com/gitlab-org/gitlab/-/issues/335214
- https://gitlab.com/gitlab-org/gitlab/-/issues/335211

Related discussion in :

-
https://gitlab.com/gitlab-com/gl-infra/production/-/issues/5069#note_618889225

lib/gitlab/database/postgres_index.rb

@@ -44,6 +44,10 @@ module Gitlab
         strong_memoize(:bloat_size) { bloat_estimate&.bloat_size || 0 }
       end
 
+      def relative_bloat_level
+        bloat_size / ondisk_size_bytes.to_f
+      end
+
       def to_s
         name
       end

lib/gitlab/database/reindexing/index_selection.rb

@@ -6,6 +6,12 @@ module Gitlab
       class IndexSelection
         include Enumerable
 
+        # Only reindex indexes with a relative bloat level (bloat estimate / size) higher than this
+        MINIMUM_RELATIVE_BLOAT = 0.2
+
+        # Only consider indexes with a total ondisk size in this range (before reindexing)
+        INDEX_SIZE_RANGE = (1.gigabyte..100.gigabyte).freeze
+
         delegate :each, to: :indexes
 
         def initialize(candidates)
@@ -24,11 +30,12 @@ module Gitlab
           # we force a N+1 pattern here and estimate bloat on a per-index
           # basis.
 
-          @indexes ||= filter_candidates.sort_by(&:bloat_size).reverse
-        end
-
-        def filter_candidates
-          candidates.not_recently_reindexed
+          @indexes ||= candidates
+            .not_recently_reindexed
+            .where(ondisk_size_bytes: INDEX_SIZE_RANGE)
+            .sort_by(&:relative_bloat_level) # forced N+1
+            .reverse
+            .select { |candidate| candidate.relative_bloat_level >= MINIMUM_RELATIVE_BLOAT }
         end
       end
     end

spec/lib/gitlab/database/postgres_index_spec.rb

@@ -97,6 +97,16 @@ RSpec.describe Gitlab::Database::PostgresIndex do
     end
   end
 
+  describe '#relative_bloat_level' do
+    subject { build(:postgres_index, bloat_estimate: bloat_estimate, ondisk_size_bytes: 1024) }
+
+    let(:bloat_estimate) { build(:postgres_index_bloat_estimate, bloat_size: 256) }
+
+    it 'calculates the relative bloat level' do
+      expect(subject.relative_bloat_level).to eq(0.25)
+    end
+  end
+
   describe '#unique?' do
     it 'returns true for a unique index' do
       expect(find('public.bar_key')).to be_unique

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

@@ -10,20 +10,50 @@ RSpec.describe Gitlab::Database::Reindexing::IndexSelection do
   before do
     swapout_view_for_table(:postgres_index_bloat_estimates)
     swapout_view_for_table(:postgres_indexes)
+
+    create_list(:postgres_index, 10, ondisk_size_bytes: 10.gigabytes).each_with_index do |index, i|
+      create(:postgres_index_bloat_estimate, index: index, bloat_size_bytes: 2.gigabyte * (i + 1))
+    end
   end
 
   def execute(sql)
     ActiveRecord::Base.connection.execute(sql)
   end
 
-  it 'orders by highest bloat first' do
-    create_list(:postgres_index, 10).each_with_index do |index, i|
-      create(:postgres_index_bloat_estimate, index: index, bloat_size_bytes: 1.megabyte * i)
+  it 'orders by highest relative bloat first' do
+    expected = Gitlab::Database::PostgresIndex.all.sort_by(&:relative_bloat_level).reverse.map(&:name)
+
+    expect(subject.map(&:name)).to eq(expected)
+  end
+
+  it 'excludes indexes with a relative bloat level below 20%' do
+    excluded = create(
+      :postgres_index_bloat_estimate,
+      index: create(:postgres_index, ondisk_size_bytes: 10.gigabytes),
+      bloat_size_bytes: 1.9.gigabyte  # 19% relative index bloat
+    )
+
+    expect(subject).not_to include(excluded.index)
   end
 
-    expected = Gitlab::Database::PostgresIndexBloatEstimate.order(bloat_size_bytes: :desc).map(&:index)
+  it 'excludes indexes smaller than 1 GB ondisk size' do
+    excluded = create(
+      :postgres_index_bloat_estimate,
+      index: create(:postgres_index, ondisk_size_bytes: 0.99.gigabytes),
+      bloat_size_bytes: 0.8.gigabyte
+    )
 
-    expect(subject).to eq(expected)
+    expect(subject).not_to include(excluded.index)
+  end
+
+  it 'excludes indexes larger than 100 GB ondisk size' do
+    excluded = create(
+      :postgres_index_bloat_estimate,
+      index: create(:postgres_index, ondisk_size_bytes: 101.gigabytes),
+      bloat_size_bytes: 25.gigabyte
+    )
+
+    expect(subject).not_to include(excluded.index)
   end
 
   context 'with time frozen' do
@@ -31,20 +61,17 @@ RSpec.describe Gitlab::Database::Reindexing::IndexSelection do
       freeze_time { example.run }
     end
 
-    it 'does not return indexes with reindex action in the last 7 days' do
-      not_recently_reindexed = create_list(:postgres_index, 2).each_with_index do |index, i|
-        create(:postgres_index_bloat_estimate, index: index, bloat_size_bytes: 1.megabyte * i)
-        create(:reindex_action, index: index, action_end: Time.zone.now - 7.days - 1.minute)
+    it 'does not return indexes with reindex action in the last 10 days' do
+      not_recently_reindexed = Gitlab::Database::PostgresIndex.all.each do |index|
+        create(:reindex_action, index: index, action_end: Time.zone.now - 10.days - 1.minute)
       end
 
-      create_list(:postgres_index, 2).each_with_index do |index, i|
-        create(:postgres_index_bloat_estimate, index: index, bloat_size_bytes: 1.megabyte * i)
+      create_list(:postgres_index, 10, ondisk_size_bytes: 10.gigabytes).each_with_index do |index, i|
+        create(:postgres_index_bloat_estimate, index: index, bloat_size_bytes: 2.gigabyte * (i + 1))
         create(:reindex_action, index: index, action_end: Time.zone.now)
       end
 
-      expected = Gitlab::Database::PostgresIndexBloatEstimate.where(identifier: not_recently_reindexed.map(&:identifier)).map(&:index).map(&:identifier).sort
-
-      expect(subject.map(&:identifier).sort).to eq(expected)
+      expect(subject.map(&:name).sort).to eq(not_recently_reindexed.map(&:name).sort)
     end
   end
 end