Disallow id field for batch counting

Using id field of the same table wouldn't make sense except in an
outer join

lib/gitlab/database/batch_count.rb

@@ -37,6 +37,7 @@ module Gitlab
       MIN_REQUIRED_BATCH_SIZE = 1_250
       MAX_ALLOWED_LOOPS = 10_000
       SLEEP_TIME_IN_SECONDS = 0.01 # 10 msec sleep
+      ALLOWED_MODES = [:itself, :distinct].freeze
 
       # Each query should take < 500ms https://gitlab.com/gitlab-org/gitlab/-/merge_requests/22705
       DEFAULT_DISTINCT_BATCH_SIZE = 10_000
@@ -55,8 +56,8 @@ 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
+
+        check_mode!(mode)
 
         # non-distinct have better performance
         batch_size ||= mode == :distinct ? DEFAULT_DISTINCT_BATCH_SIZE : DEFAULT_BATCH_SIZE
@@ -102,6 +103,12 @@ module Gitlab
       def actual_finish(finish)
         finish || @relation.maximum(@column) || 0
       end
+
+      def check_mode!(mode)
+        raise "The mode #{mode.inspect} is not supported" unless ALLOWED_MODES.include?(mode)
+        raise 'Use distinct count for optimized distinct counting' if @relation.limit(1).distinct_value.present? && mode != :distinct
+        raise 'Use distinct count only with non id fields' if @column == :id && mode == :distinct
+      end
     end
   end
 end

spec/lib/gitlab/database/batch_count_spec.rb

@@ -86,10 +86,6 @@ describe Gitlab::Database::BatchCount do
   end
 
   describe '#batch_distinct_count' do
-    it 'counts with :id field' do
-      expect(described_class.batch_distinct_count(model, :id)).to eq(5)
-    end
-
     it 'counts with column field' do
       expect(described_class.batch_distinct_count(model, column)).to eq(2)
     end
@@ -137,6 +133,12 @@ describe Gitlab::Database::BatchCount do
       it 'returns fallback if batch size is less than min required' do
         expect(described_class.batch_distinct_count(model, column, batch_size: small_batch_size)).to eq(fallback)
       end
+
+      it 'will raise an error if distinct count with the :id column is requested' do
+        expect do
+          described_class.batch_count(described_class.batch_distinct_count(model, :id))
+        end.to raise_error 'Use distinct count only with non id fields'
+      end
     end
   end
 end