Optimize issue rebalancing UPDATE query

This change updates the issue rebalancing UPDATE query by sorting the
updatable issues by id. This will improve the data locality a bit and
probably speed up random I/O.

app/services/issue_rebalancing_service.rb

@@ -17,10 +17,16 @@ class IssueRebalancingService
 
     start = RelativePositioning::START_POSITION - (gaps / 2) * gap_size
 
+    if Feature.enabled?(:issue_rebalancing_optimization)
+      Issue.transaction do
+        sort_pairs_by_id(start).each_slice(100) { |pairs| assign_positions_without_position_calculation(pairs) }
+      end
+    else
       Issue.transaction do
         indexed_ids.each_slice(100) { |pairs| assign_positions(start, pairs) }
       end
     end
+  end
 
   private
 
@@ -32,13 +38,30 @@ class IssueRebalancingService
   end
   # rubocop: enable CodeReuse/ActiveRecord
 
-  # rubocop: disable CodeReuse/ActiveRecord
+  def sort_pairs_by_id(start)
+    indexed_ids.map do |id, index|
+      [id, start + (index * gap_size)]
+    end.sort_by(&:first)
+  end
+
+  def assign_positions_without_position_calculation(pairs_with_position)
+    values = pairs_with_position.map do |id, index|
+      "(#{id}, #{index})"
+    end.join(', ')
+
+    run_update_query(values)
+  end
+
   def assign_positions(start, positions)
     values = positions.map do |id, index|
       "(#{id}, #{start + (index * gap_size)})"
     end.join(', ')
 
-    Issue.connection.exec_query(<<~SQL, "rebalance issue positions")
+    run_update_query(values)
+  end
+
+  def run_update_query(values)
+    Issue.connection.exec_query(<<~SQL, "rebalance issue positions batches ordered by id")
       WITH cte(cte_id, new_pos) AS (
        SELECT *
        FROM (VALUES #{values}) as t (id, pos)
@@ -49,7 +72,6 @@ class IssueRebalancingService
       WHERE cte_id = id
     SQL
   end
-  # rubocop: enable CodeReuse/ActiveRecord
 
   def issue_count
     @issue_count ||= base.count

config/feature_flags/development/issue_rebalancing_optimization.yml

+---
+name: issue_rebalancing_optimization
+introduced_by_url:
+rollout_issue_url:
+milestone: '13.9'
+type: development
+group: group::plan
+default_enabled: false

spec/services/issue_rebalancing_service_spec.rb

@@ -32,6 +32,7 @@ RSpec.describe IssueRebalancingService do
     project.reload.issues.reorder(relative_position: :asc).to_a
   end
 
+  shared_examples 'IssueRebalancingService shared examples' do
     it 'rebalances a set of issues with clumps at the end and start' do
       all_issues = start_clump + unclumped + end_clump.reverse
       service = described_class.new(project.issues.first)
@@ -98,4 +99,21 @@ RSpec.describe IssueRebalancingService do
 
       expect { described_class.new(issue).execute }.to raise_error(described_class::TooManyIssues)
     end
+  end
+
+  context 'when issue_rebalancing_optimization feature flag is on' do
+    before do
+      stub_feature_flags(issue_rebalancing_optimization: true)
+    end
+
+    it_behaves_like 'IssueRebalancingService shared examples'
+  end
+
+  context 'when issue_rebalancing_optimization feature flag is on' do
+    before do
+      stub_feature_flags(issue_rebalancing_optimization: false)
+    end
+
+    it_behaves_like 'IssueRebalancingService shared examples'
+  end
 end