Disable ci_builds reindexing on Sundays

lib/gitlab/database/reindexing/index_selection.rb

@@ -12,6 +12,10 @@ class IndexSelection
         # Only consider indexes beyond this size (before reindexing)
         INDEX_SIZE_MINIMUM = 1.gigabyte
 
+        VERY_LARGE_TABLES = %i[
+          ci_builds
+        ].freeze
+
         delegate :each, to: :indexes
 
         def initialize(candidates)
@@ -30,13 +34,24 @@ def indexes
           # we force a N+1 pattern here and estimate bloat on a per-index
           # basis.
 
-          @indexes ||= candidates
-            .not_recently_reindexed
-            .where('ondisk_size_bytes >= ?', INDEX_SIZE_MINIMUM)
+          @indexes ||= relations_that_need_cleaning_before_deadline
             .sort_by(&:relative_bloat_level) # forced N+1
             .reverse
             .select { |candidate| candidate.relative_bloat_level >= MINIMUM_RELATIVE_BLOAT }
         end
+
+        def relations_that_need_cleaning_before_deadline
+          relation = candidates.not_recently_reindexed.where('ondisk_size_bytes >= ?', INDEX_SIZE_MINIMUM)
+          relation = relation.where.not(tablename: VERY_LARGE_TABLES) if too_late_for_very_large_table?
+          relation
+        end
+
+        # The reindexing process takes place during the weekends and starting a
+        # reindexing action on a large table late on Sunday could span during
+        # Monday. We don't want this because it prevents vacuum from running.
+        def too_late_for_very_large_table?
+          Date.today.sunday?
+        end
       end
     end
   end

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

@@ -74,4 +74,22 @@ def execute(sql)
       expect(subject.map(&:name).sort).to eq(not_recently_reindexed.map(&:name).sort)
     end
   end
+
+  context 'with restricted tables' do
+    let!(:ci_builds) do
+      create(
+        :postgres_index_bloat_estimate,
+        index: create(:postgres_index, ondisk_size_bytes: 100.gigabytes, tablename: 'ci_builds'),
+        bloat_size_bytes: 20.gigabyte
+      )
+    end
+
+    context 'when executed on Saturdays', time_travel_to: '2022-12-17T09:44:07Z' do
+      it { expect(subject).to include(ci_builds.index) }
+    end
+
+    context 'when executed on Sundays', time_travel_to: '2022-12-18T09:44:07Z' do
+      it { expect(subject).not_to include(ci_builds.index) }
+    end
+  end
 end