Add UNION optimization for keyset pagination

- Add UNION optimization for keyset pagination
- Add an EachBatch like iterator class

app/models/concerns/from_set_operator.rb

@@ -10,8 +10,8 @@ module FromSetOperator
 
     raise "Trying to redefine method '#{method(method_name)}'" if methods.include?(method_name)
 
-    define_method(method_name) do |members, remove_duplicates: true, alias_as: table_name|
-      operator_sql = operator.new(members, remove_duplicates: remove_duplicates).to_sql
+    define_method(method_name) do |members, remove_duplicates: true, remove_order: true, alias_as: table_name|
+      operator_sql = operator.new(members, remove_duplicates: remove_duplicates, remove_order: remove_order).to_sql
 
       from(Arel.sql("(#{operator_sql}) #{alias_as}"))
     end

app/models/issue.rb

@@ -108,6 +108,7 @@ class Issue < ApplicationRecord
   scope :order_due_date_desc, -> { reorder(::Gitlab::Database.nulls_last_order('due_date', 'DESC')) }
   scope :order_closest_future_date, -> { reorder(Arel.sql('CASE WHEN issues.due_date >= CURRENT_DATE THEN 0 ELSE 1 END ASC, ABS(CURRENT_DATE - issues.due_date) ASC')) }
   scope :order_relative_position_asc, -> { reorder(::Gitlab::Database.nulls_last_order('relative_position', 'ASC')) }
+  scope :order_relative_position_desc, -> { reorder(::Gitlab::Database.nulls_first_order('relative_position', 'DESC')) }
   scope :order_closed_date_desc, -> { reorder(closed_at: :desc) }
   scope :order_created_at_desc, -> { reorder(created_at: :desc) }
   scope :order_severity_asc, -> { includes(:issuable_severity).order('issuable_severities.severity ASC NULLS FIRST') }

lib/gitlab/database.rb

@@ -123,6 +123,16 @@ module Gitlab
       # ignore - happens when Rake tasks yet have to create a database, e.g. for testing
     end
 
+    def self.nulls_order(field, direction = :asc, nulls_order = :nulls_last)
+      raise ArgumentError unless [:nulls_last, :nulls_first].include?(nulls_order)
+      raise ArgumentError unless [:asc, :desc].include?(direction)
+
+      case nulls_order
+      when :nulls_last then nulls_last_order(field, direction)
+      when :nulls_first then nulls_first_order(field, direction)
+      end
+    end
+
     def self.nulls_last_order(field, direction = 'ASC')
       Arel.sql("#{field} #{direction} NULLS LAST")
     end

lib/gitlab/pagination/keyset/iterator.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module Pagination
+    module Keyset
+      class Iterator
+        def initialize(scope:, use_union_optimization: false)
+          @scope = scope
+          @order = Gitlab::Pagination::Keyset::Order.extract_keyset_order_object(scope)
+          @use_union_optimization = use_union_optimization
+        end
+
+        # rubocop: disable CodeReuse/ActiveRecord
+        def each_batch(of: 1000)
+          cursor_attributes = {}
+
+          loop do
+            current_scope = scope.dup.limit(of)
+            relation = order
+              .apply_cursor_conditions(current_scope, cursor_attributes, { use_union_optimization: @use_union_optimization })
+              .reorder(order)
+              .limit(of)
+
+            yield relation
+
+            last_record = relation.last
+            break unless last_record
+
+            cursor_attributes = order.cursor_attributes_for_node(last_record)
+          end
+        end
+        # rubocop: enable CodeReuse/ActiveRecord
+
+        private
+
+        attr_reader :scope, :order
+      end
+    end
+  end
+end

lib/gitlab/pagination/keyset/order.rb

@@ -135,7 +135,7 @@ module Gitlab
         #
         # (id < 3 AND created_at IS NULL) OR (created_at IS NOT NULL)
         def build_where_values(values)
-          return if values.blank?
+          return [] if values.blank?
 
           verify_incoming_values!(values)
 
@@ -156,13 +156,26 @@ module Gitlab
             end
           end
 
-          build_or_query(where_values)
+          where_values
+        end
+
+        def where_values_with_or_query(values)
+          build_or_query(build_where_values(values.with_indifferent_access))
         end
 
         # rubocop: disable CodeReuse/ActiveRecord
-        def apply_cursor_conditions(scope, values = {})
+        def apply_cursor_conditions(scope, values = {}, options = { use_union_optimization: false })
+          values ||= {}
+          transformed_values = values.with_indifferent_access
           scope = apply_custom_projections(scope)
-          scope.where(build_where_values(values.with_indifferent_access))
+
+          where_values = build_where_values(transformed_values)
+
+          if options[:use_union_optimization] && where_values.size > 1
+            build_union_query(scope, where_values).reorder(self)
+          else
+            scope.where(build_or_query(where_values)) # rubocop: disable CodeReuse/ActiveRecord
+          end
         end
         # rubocop: enable CodeReuse/ActiveRecord
 
@@ -212,11 +225,19 @@ module Gitlab
         end
 
         def build_or_query(expressions)
-          or_expression = expressions.reduce { |or_expression, expression| Arel::Nodes::Or.new(or_expression, expression) }
+          return [] if expressions.blank?
 
+          or_expression = expressions.reduce { |or_expression, expression| Arel::Nodes::Or.new(or_expression, expression) }
           Arel::Nodes::Grouping.new(or_expression)
         end
 
+        def build_union_query(scope, where_values)
+          scopes = where_values.map do |where_value|
+            scope.dup.where(where_value).reorder(self) # rubocop: disable CodeReuse/ActiveRecord
+          end
+          scope.model.from_union(scopes, remove_duplicates: false, remove_order: false)
+        end
+
         def to_sql_literal(column_definitions)
           column_definitions.map do |column_definition|
             if column_definition.order_expression.respond_to?(:to_sql)

spec/factories/issues.rb

@@ -13,6 +13,14 @@ FactoryBot.define do
       confidential { true }
     end
 
+    trait :with_asc_relative_position do
+      sequence(:relative_position) { |n| n * 1000 }
+    end
+
+    trait :with_desc_relative_position do
+      sequence(:relative_position) { |n| -n * 1000 }
+    end
+
     trait :opened do
       state_id { Issue.available_states[:opened] }
     end

spec/lib/gitlab/pagination/keyset/iterator_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Gitlab::Pagination::Keyset::Iterator do
+  let_it_be(:project) { create(:project) }
+  let_it_be(:issue_list_with_same_pos) { create_list(:issue, 3, project: project, relative_position: 100, updated_at: 1.day.ago) }
+  let_it_be(:issue_list_with_null_pos) { create_list(:issue, 3, project: project, relative_position: nil, updated_at: 1.day.ago) }
+  let_it_be(:issue_list_with_asc_pos) { create_list(:issue, 3, :with_asc_relative_position, project: project, updated_at: 1.day.ago) }
+
+  let(:klass) { Issue }
+  let(:column) { 'relative_position' }
+  let(:direction) { :asc }
+  let(:reverse_direction) { ::Gitlab::Pagination::Keyset::ColumnOrderDefinition::REVERSED_ORDER_DIRECTIONS[direction] }
+  let(:nulls_position) { :nulls_last }
+  let(:reverse_nulls_position) { ::Gitlab::Pagination::Keyset::ColumnOrderDefinition::REVERSED_NULL_POSITIONS[nulls_position] }
+  let(:custom_reorder) do
+    Gitlab::Pagination::Keyset::Order.build([
+      Gitlab::Pagination::Keyset::ColumnOrderDefinition.new(
+        attribute_name: column,
+         column_expression: klass.arel_table[column],
+         order_expression: ::Gitlab::Database.nulls_order(column, direction, nulls_position),
+         reversed_order_expression: ::Gitlab::Database.nulls_order(column, reverse_direction, reverse_nulls_position),
+         order_direction: direction,
+         nullable: nulls_position,
+         distinct: false
+      ),
+      Gitlab::Pagination::Keyset::ColumnOrderDefinition.new(
+        attribute_name: 'id',
+         order_expression: klass.arel_table[:id].send(direction),
+         add_to_projections: true
+      )
+    ])
+  end
+
+  let(:scope) { project.issues.reorder(custom_reorder) }
+
+  subject { described_class.new(scope: scope) }
+
+  describe '.each_batch' do
+    it 'yields an ActiveRecord::Relation when a block is given' do
+      subject.each_batch(of: 1) do |relation|
+        expect(relation).to be_a_kind_of(ActiveRecord::Relation)
+      end
+    end
+
+    it 'accepts a custom batch size' do
+      count = 0
+
+      subject.each_batch(of: 2) { |relation| count += relation.count(:all) }
+
+      expect(count).to eq(9)
+    end
+
+    it 'allows updating of the yielded relations' do
+      time = Time.current
+
+      subject.each_batch(of: 2) do |relation|
+        relation.update_all(updated_at: time)
+      end
+
+      expect(Issue.where(updated_at: time).count).to eq(9)
+    end
+
+    context 'with ordering direction' do
+      context 'when ordering asc' do
+        it 'orders ascending by default, including secondary order column' do
+          positions = []
+
+          subject.each_batch(of: 2) { |rel| positions.concat(rel.pluck(:relative_position, :id)) }
+
+          expect(positions).to eq(project.issues.order_relative_position_asc.order(id: :asc).pluck(:relative_position, :id))
+        end
+      end
+
+      context 'when reversing asc order' do
+        let(:scope) { project.issues.order(custom_reorder.reversed_order) }
+
+        it 'orders in reverse of ascending' do
+          positions = []
+
+          subject.each_batch(of: 2) { |rel| positions.concat(rel.pluck(:relative_position, :id)) }
+
+          expect(positions).to eq(project.issues.order_relative_position_desc.order(id: :desc).pluck(:relative_position, :id))
+        end
+      end
+
+      context 'when asc order, with nulls first' do
+        let(:nulls_position) { :nulls_first }
+
+        it 'orders ascending with nulls first' do
+          positions = []
+
+          subject.each_batch(of: 2) { |rel| positions.concat(rel.pluck(:relative_position, :id)) }
+
+          expect(positions).to eq(project.issues.reorder(::Gitlab::Database.nulls_first_order('relative_position', 'ASC')).order(id: :asc).pluck(:relative_position, :id))
+        end
+      end
+
+      context 'when ordering desc' do
+        let(:direction) { :desc }
+        let(:nulls_position) { :nulls_last }
+
+        it 'orders descending' do
+          positions = []
+
+          subject.each_batch(of: 2) { |rel| positions.concat(rel.pluck(:relative_position, :id)) }
+
+          expect(positions).to eq(project.issues.reorder(::Gitlab::Database.nulls_last_order('relative_position', 'DESC')).order(id: :desc).pluck(:relative_position, :id))
+        end
+      end
+
+      context 'when ordering by columns are repeated twice' do
+        let(:direction) { :desc }
+        let(:column) { :id }
+
+        it 'orders descending' do
+          positions = []
+
+          subject.each_batch(of: 2) { |rel| positions.concat(rel.pluck(:id)) }
+
+          expect(positions).to eq(project.issues.reorder(id: :desc).pluck(:id))
+        end
+      end
+    end
+  end
+end