Improve finding of terminus when shifting items

We already have the min / max position from the previous query so we
just need to find the object with that position instead of querying and
sorting all objects again and finding the min / max one.

Changelog: performance

app/models/concerns/relative_positioning.rb

@@ -53,13 +53,18 @@ module RelativePositioning
 
       return [size, starting_from] if size >= MIN_GAP
 
+      terminus =
+        if Feature.enabled?(:optimize_shifting_relative_positions, default_enabled: :yaml)
+          context.at_position(starting_from)
+        else
+          at_end ? context.max_sibling : context.min_sibling
+        end
+
       if at_end
-        terminus = context.max_sibling
         terminus.shift_left
         max_relative_position = terminus.relative_position
         [[(MAX_POSITION - max_relative_position) / gaps, IDEAL_DISTANCE].min, max_relative_position]
       else
-        terminus = context.min_sibling
         terminus.shift_right
         min_relative_position = terminus.relative_position
         [[(min_relative_position - MIN_POSITION) / gaps, IDEAL_DISTANCE].min, min_relative_position]

changelogs/unreleased/improve-finding-of-terminus-when-shifting-positions.yml

+---
+title: Improve shifting of positions when creating issues
+merge_request: 59745
+author:
+type: performance

config/feature_flags/development/optimize_shifting_relative_positions.yml

+---
+name: optimize_shifting_relative_positions
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/59745
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/328402
+milestone: '13.12'
+type: development
+group: group::project management
+default_enabled: false

lib/gitlab/relative_positioning.rb

@@ -13,6 +13,7 @@ module Gitlab
     MIN_GAP = 2
 
     NoSpaceLeft = Class.new(StandardError)
+    InvalidPosition = Class.new(StandardError)
     IllegalRange = Class.new(ArgumentError)
 
     def self.range(lhs, rhs)

lib/gitlab/relative_positioning/item_context.rb

@@ -129,6 +129,14 @@ module Gitlab
         neighbour(sib)
       end
 
+      def at_position(position)
+        item = scoped_items.find_by(relative_position: position)
+
+        raise InvalidPosition, 'No item found at the specified position' if item.nil?
+
+        neighbour(item)
+      end
+
       def shift_left
         move_sequence_before(true)
         object.reset_relative_position

spec/lib/gitlab/relative_positioning/item_context_spec.rb

@@ -212,4 +212,20 @@ RSpec.describe Gitlab::RelativePositioning::ItemContext do
       end
     end
   end
+
+  describe '#at_position' do
+    let_it_be(:issue) { create_issue(500) }
+    let_it_be(:issue_2) { create_issue(510) }
+
+    let(:subject) { described_class.new(issue, range) }
+
+    it 'finds the item at the specified position' do
+      expect(subject.at_position(500)).to eq(described_class.new(issue, range))
+      expect(subject.at_position(510)).to eq(described_class.new(issue_2, range))
+    end
+
+    it 'raises InvalidPosition when the item cannot be found' do
+      expect { subject.at_position(501) }.to raise_error Gitlab::RelativePositioning::InvalidPosition
+    end
+  end
 end

spec/support/shared_examples/models/relative_positioning_shared_examples.rb

@@ -66,7 +66,7 @@ RSpec.shared_examples 'a class that supports relative positioning' do
     end
   end
 
-  describe '.move_nulls_to_end' do
+  shared_examples '.move_nulls_to_end' do
     let(:item3) { create_item }
     let(:sibling_query) { item1.class.relative_positioning_query_base(item1) }
 
@@ -186,7 +186,7 @@ RSpec.shared_examples 'a class that supports relative positioning' do
     end
   end
 
-  describe '.move_nulls_to_start' do
+  shared_examples '.move_nulls_to_start' do
     let(:item3) { create_item }
     let(:sibling_query) { item1.class.relative_positioning_query_base(item1) }
 
@@ -261,6 +261,24 @@ RSpec.shared_examples 'a class that supports relative positioning' do
     end
   end
 
+  context 'when optimize_shifting_relative_positions is enabled' do
+    before do
+      stub_feature_flags(optimize_shifting_relative_positions: true)
+    end
+
+    it_behaves_like '.move_nulls_to_start'
+    it_behaves_like '.move_nulls_to_end'
+  end
+
+  context 'when optimize_shifting_relative_positions is disabled' do
+    before do
+      stub_feature_flags(optimize_shifting_relative_positions: false)
+    end
+
+    it_behaves_like '.move_nulls_to_start'
+    it_behaves_like '.move_nulls_to_end'
+  end
+
   describe '#move_before' do
     let(:item3) { create(factory, default_params) }