Fix counting of groups in admin dashboard

1. Ignore tables that use STI in reltuples count strategy.

   Models that use Rails' single-type inheritance, such as `Group` and
   `CiService`, need an additional WHERE clause to count the total
   properly, which isn't supported by the reltuples strategy.  For now,
   we just omit these from the statistics sampling and rely on the other
   strategies to get this data.

2. Fix tablesample count strategy not counting groups properly.

   Models such as `Group` needs a WHERE clause to distinguish it from
   namespaces. We now add in the WHERE clause if STI is in use.

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/7435

changelogs/unreleased/sh-skip-sti-tables-reltuples.yml

+---
+title: Fix counting of groups in admin dashboard
+merge_request: 26009
+author:
+type: fixed

lib/gitlab/database/count/reltuples_count_strategy.rb

@@ -37,6 +37,22 @@ module Gitlab
 
         private
 
+        # Models using single-type inheritance (STI) don't work with
+        # reltuple count estimates. We just have to ignore them and
+        # use another strategy to compute them.
+        def non_sti_models
+          models.reject { |model| sti_model?(model) }
+        end
+
+        def non_sti_table_names
+          non_sti_models.map(&:table_name)
+        end
+
+        def sti_model?(model)
+          model.column_names.include?(model.inheritance_column) &&
+            model.base_class != model
+        end
+
         def table_names
           models.map(&:table_name)
         end
@@ -47,7 +63,7 @@ module Gitlab
           # Querying tuple stats only works on the primary. Due to load balancing, the
           # easiest way to do this is to start a transaction.
           ActiveRecord::Base.transaction do
-            get_statistics(table_names, check_statistics: check_statistics).each_with_object({}) do |row, data|
+            get_statistics(non_sti_table_names, check_statistics: check_statistics).each_with_object({}) do |row, data|
               model = table_to_model[row.table_name]
               data[model] = row.estimate
             end

lib/gitlab/database/count/tablesample_count_strategy.rb

@@ -48,12 +48,20 @@ module Gitlab
           end
         end
 
+        def where_clause(model)
+          return unless sti_model?(model)
+
+          "WHERE #{model.inheritance_column} = '#{model.name}'"
+        end
+
         def tablesample_count(model, estimate)
           portion = (TABLESAMPLE_ROW_TARGET.to_f / estimate).round(4)
           inverse = 1 / portion
           query = <<~SQL
             SELECT (COUNT(*)*#{inverse})::integer AS count
-            FROM #{model.table_name} TABLESAMPLE SYSTEM (#{portion * 100})
+            FROM #{model.table_name}
+            TABLESAMPLE SYSTEM (#{portion * 100})
+            #{where_clause(model)}
           SQL
 
           rows = ActiveRecord::Base.connection.select_all(query)

spec/lib/gitlab/database/count/reltuples_count_strategy_spec.rb

@@ -6,10 +6,11 @@ describe Gitlab::Database::Count::ReltuplesCountStrategy do
     create(:identity)
   end
 
-  let(:models) { [Project, Identity] }
   subject { described_class.new(models).count }
 
   describe '#count', :postgresql do
+    let(:models) { [Project, Identity] }
+
     context 'when reltuples is up to date' do
       before do
         ActiveRecord::Base.connection.execute('ANALYZE projects')
@@ -23,6 +24,22 @@ describe Gitlab::Database::Count::ReltuplesCountStrategy do
       end
     end
 
+    context 'when models using single-type inheritance are used' do
+      let(:models) { [Group, CiService, Namespace] }
+
+      before do
+        models.each do |model|
+          ActiveRecord::Base.connection.execute("ANALYZE #{model.table_name}")
+        end
+      end
+
+      it 'returns nil counts for inherited tables' do
+        models.each { |model| expect(model).not_to receive(:count) }
+
+        expect(subject).to eq({ Namespace => 3 })
+      end
+    end
+
     context 'insufficient permissions' do
       it 'returns an empty hash' do
         allow(ActiveRecord::Base).to receive(:transaction).and_raise(PG::InsufficientPrivilege)

spec/lib/gitlab/database/count/tablesample_count_strategy_spec.rb

@@ -4,15 +4,23 @@ describe Gitlab::Database::Count::TablesampleCountStrategy do
   before do
     create_list(:project, 3)
     create(:identity)
+    create(:group)
   end
 
-  let(:models) { [Project, Identity] }
+  let(:models) { [Project, Identity, Group, Namespace] }
   let(:strategy) { described_class.new(models) }
 
   subject { strategy.count }
 
   describe '#count', :postgresql do
-    let(:estimates) { { Project => threshold + 1, Identity => threshold - 1 } }
+    let(:estimates) do
+      {
+        Project => threshold + 1,
+        Identity => threshold - 1,
+        Group => threshold + 1,
+        Namespace => threshold + 1
+      }
+    end
     let(:threshold) { Gitlab::Database::Count::TablesampleCountStrategy::EXACT_COUNT_THRESHOLD }
 
     before do
@@ -30,9 +38,13 @@ describe Gitlab::Database::Count::TablesampleCountStrategy do
     context 'for tables with an estimated large size' do
       it 'performs a tablesample count' do
         expect(Project).not_to receive(:count)
+        expect(Group).not_to receive(:count)
+        expect(Namespace).not_to receive(:count)
 
         result = subject
         expect(result[Project]).to eq(3)
+        expect(result[Group]).to eq(1)
+        expect(result[Namespace]).to eq(4)
       end
     end