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

2 files changed, 32 insertions, 3 deletions

diff --git a/spec/lib/gitlab/database/count/reltuples_count_strategy_spec.rb b/spec/lib/gitlab/database/count/reltuples_count_strategy_spec.rb
index b44e8c5a110..bd3c66d0548 100644
--- a/spec/lib/gitlab/database/count/reltuples_count_strategy_spec.rb
+++ b/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)
diff --git a/spec/lib/gitlab/database/count/tablesample_count_strategy_spec.rb b/spec/lib/gitlab/database/count/tablesample_count_strategy_spec.rb
index 203f9344a41..40d810b195b 100644
--- a/spec/lib/gitlab/database/count/tablesample_count_strategy_spec.rb
+++ b/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