Disallow distinct count for regular batch count

Distinct counts should use distinct_count method or mode optimized for
slower distinct counting.

changelogs/unreleased/213062-disallow-distinct-count-for-the-ordinary-batch-counter.yml

+---
+title: Disallow distinct count for regular batch count
+merge_request: 28518
+author:
+type: performance

lib/gitlab/database/batch_count.rb

@@ -56,6 +56,7 @@ module Gitlab
       def count(batch_size: nil, mode: :itself, start: nil, finish: nil)
         raise 'BatchCount can not be run inside a transaction' if ActiveRecord::Base.connection.transaction_open?
         raise "The mode #{mode.inspect} is not supported" unless [:itself, :distinct].include?(mode)
+        raise 'Use distinct count for optimized distinct counting' if @relation.limit(1).distinct_value.present? && mode != :distinct
 
         # non-distinct have better performance
         batch_size ||= mode == :distinct ? DEFAULT_DISTINCT_BATCH_SIZE : DEFAULT_BATCH_SIZE

spec/lib/gitlab/database/batch_count_spec.rb

@@ -48,6 +48,12 @@ describe Gitlab::Database::BatchCount do
       [1, 2, 4, 5, 6].each { |i| expect(described_class.batch_count(model, batch_size: i)).to eq(5) }
     end
 
+    it 'will raise an error if distinct count is requested' do
+      expect do
+        described_class.batch_count(model.distinct(column))
+      end.to raise_error 'Use distinct count for optimized distinct counting'
+    end
+
     context 'in a transaction' do
       let(:in_transaction) { true }