Fix Error 500 viewing admin page due to statement timeouts

Uses PostgreSQL tuple estimates to provide a much faster yet approximate
count. See https://wiki.postgresql.org/wiki/Slow_Counting for more details.
We only use this fast method if the table has been analyzed or vacuumed
within the last hour.

Closes #46255

1 files changed, 62 insertions, 0 deletions

diff --git a/spec/lib/gitlab/database/count_spec.rb b/spec/lib/gitlab/database/count_spec.rb
new file mode 100644
index 00000000000..9d9caaabe16
--- /dev/null
+++ b/spec/lib/gitlab/database/count_spec.rb
@@ -0,0 +1,62 @@
+require 'spec_helper'
+
+describe Gitlab::Database::Count do
+  before do
+    create_list(:project, 3)
+  end
+
+  describe '.execute_estimate_if_updated_recently', :postgresql do
+    context 'when reltuples have not been updated' do
+      before do
+        expect(described_class).to receive(:reltuples_updated_recently?).and_return(false)
+      end
+
+      it 'returns nil' do
+        expect(described_class.execute_estimate_if_updated_recently(Project)).to be nil
+      end
+    end
+
+    context 'when reltuples have been updated' do
+      before do
+        ActiveRecord::Base.connection.execute('ANALYZE projects')
+      end
+
+      it 'calls postgresql_estimate_query' do
+        expect(described_class).to receive(:postgresql_estimate_query).with(Project).and_call_original
+        expect(described_class.execute_estimate_if_updated_recently(Project)).to eq(3)
+      end
+    end
+  end
+
+  describe '.approximate_count' do
+    context 'when reltuples have not been updated' do
+      it 'counts all projects the normal way' do
+        allow(described_class).to receive(:reltuples_updated_recently?).and_return(false)
+
+        expect(Project).to receive(:count).and_call_original
+        expect(described_class.approximate_count(Project)).to eq(3)
+      end
+    end
+
+    context 'no permission' do
+      it 'falls back to standard query' do
+        allow(described_class).to receive(:reltuples_updated_recently?).and_raise(PG::InsufficientPrivilege)
+
+        expect(Project).to receive(:count).and_call_original
+        expect(described_class.approximate_count(Project)).to eq(3)
+      end
+    end
+
+    describe 'when reltuples have been updated', :postgresql do
+      before do
+        ActiveRecord::Base.connection.execute('ANALYZE projects')
+      end
+
+      it 'counts all projects in the fast way' do
+        expect(described_class).to receive(:postgresql_estimate_query).with(Project).and_call_original
+
+        expect(described_class.approximate_count(Project)).to eq(3)
+      end
+    end
+  end
+end