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

@@ -17,8 +17,14 @@ class IssueRebalancingService
 
     start = RelativePositioning::START_POSITION - (gaps / 2) * gap_size
 
-    Issue.transaction do
-      indexed_ids.each_slice(100) { |pairs| assign_positions(start, pairs) }
+    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
 
@@ -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

@@ -32,70 +32,88 @@ RSpec.describe IssueRebalancingService do
     project.reload.issues.reorder(relative_position: :asc).to_a
   end
 
-  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)
+  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)
 
-    expect { service.execute }.not_to change { issues_in_position_order.map(&:id) }
+      expect { service.execute }.not_to change { issues_in_position_order.map(&:id) }
 
-    all_issues.each(&:reset)
+      all_issues.each(&:reset)
 
-    gaps = all_issues.take(all_issues.count - 1).zip(all_issues.drop(1)).map do |a, b|
-      b.relative_position - a.relative_position
+      gaps = all_issues.take(all_issues.count - 1).zip(all_issues.drop(1)).map do |a, b|
+        b.relative_position - a.relative_position
+      end
+
+      expect(gaps).to all(be > RelativePositioning::MIN_GAP)
+      expect(all_issues.first.relative_position).to be > (RelativePositioning::MIN_POSITION * 0.9999)
+      expect(all_issues.last.relative_position).to be < (RelativePositioning::MAX_POSITION * 0.9999)
     end
 
-    expect(gaps).to all(be > RelativePositioning::MIN_GAP)
-    expect(all_issues.first.relative_position).to be > (RelativePositioning::MIN_POSITION * 0.9999)
-    expect(all_issues.last.relative_position).to be < (RelativePositioning::MAX_POSITION * 0.9999)
-  end
+    it 'is idempotent' do
+      service = described_class.new(project.issues.first)
 
-  it 'is idempotent' do
-    service = described_class.new(project.issues.first)
+      expect do
+        service.execute
+        service.execute
+      end.not_to change { issues_in_position_order.map(&:id) }
+    end
 
-    expect do
-      service.execute
-      service.execute
-    end.not_to change { issues_in_position_order.map(&:id) }
-  end
+    it 'does nothing if the feature flag is disabled' do
+      stub_feature_flags(rebalance_issues: false)
+      issue = project.issues.first
+      issue.project
+      issue.project.group
+      old_pos = issue.relative_position
 
-  it 'does nothing if the feature flag is disabled' do
-    stub_feature_flags(rebalance_issues: false)
-    issue = project.issues.first
-    issue.project
-    issue.project.group
-    old_pos = issue.relative_position
+      service = described_class.new(issue)
 
-    service = described_class.new(issue)
+      expect { service.execute }.not_to exceed_query_limit(0)
+      expect(old_pos).to eq(issue.reload.relative_position)
+    end
 
-    expect { service.execute }.not_to exceed_query_limit(0)
-    expect(old_pos).to eq(issue.reload.relative_position)
-  end
+    it 'acts if the flag is enabled for the project' do
+      issue = create(:issue, project: project, author: user, relative_position: max_pos)
+      stub_feature_flags(rebalance_issues: issue.project)
 
-  it 'acts if the flag is enabled for the project' do
-    issue = create(:issue, project: project, author: user, relative_position: max_pos)
-    stub_feature_flags(rebalance_issues: issue.project)
+      service = described_class.new(issue)
 
-    service = described_class.new(issue)
+      expect { service.execute }.to change { issue.reload.relative_position }
+    end
 
-    expect { service.execute }.to change { issue.reload.relative_position }
-  end
+    it 'acts if the flag is enabled for the group' do
+      issue = create(:issue, project: project, author: user, relative_position: max_pos)
+      project.update!(group: create(:group))
+      stub_feature_flags(rebalance_issues: issue.project.group)
 
-  it 'acts if the flag is enabled for the group' do
-    issue = create(:issue, project: project, author: user, relative_position: max_pos)
-    project.update!(group: create(:group))
-    stub_feature_flags(rebalance_issues: issue.project.group)
+      service = described_class.new(issue)
 
-    service = described_class.new(issue)
+      expect { service.execute }.to change { issue.reload.relative_position }
+    end
+
+    it 'aborts if there are too many issues' do
+      issue = project.issues.first
+      base = double(count: 10_001)
 
-    expect { service.execute }.to change { issue.reload.relative_position }
+      allow(Issue).to receive(:relative_positioning_query_base).with(issue).and_return(base)
+
+      expect { described_class.new(issue).execute }.to raise_error(described_class::TooManyIssues)
+    end
   end
 
-  it 'aborts if there are too many issues' do
-    issue = project.issues.first
-    base = double(count: 10_001)
+  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
 
-    allow(Issue).to receive(:relative_positioning_query_base).with(issue).and_return(base)
+  context 'when issue_rebalancing_optimization feature flag is on' do
+    before do
+      stub_feature_flags(issue_rebalancing_optimization: false)
+    end
 
-    expect { described_class.new(issue).execute }.to raise_error(described_class::TooManyIssues)
+    it_behaves_like 'IssueRebalancingService shared examples'
   end
 end