Use row comparison in keyset pagination

This change uses row comparison in keyset pagination when the ORDER BY
columns are not nullable and the order direction is the same.

Example: (created_at, id) > ('2012-09-18 01:40:01+00', 15)

lib/gitlab/pagination/keyset/column_order_definition.rb

@@ -120,7 +120,7 @@ module Gitlab
         AREL_ORDER_CLASSES = { Arel::Nodes::Ascending => :asc, Arel::Nodes::Descending => :desc }.freeze
         ALLOWED_NULLABLE_VALUES = [:not_nullable, :nulls_first, :nulls_last].freeze
 
-        attr_reader :attribute_name, :column_expression, :order_expression, :add_to_projections
+        attr_reader :attribute_name, :column_expression, :order_expression, :add_to_projections, :order_direction
 
         def initialize(attribute_name:, order_expression:, column_expression: nil, reversed_order_expression: nil, nullable: :not_nullable, distinct: true, order_direction: nil, add_to_projections: false)
           @attribute_name = attribute_name
@@ -175,7 +175,7 @@ module Gitlab
 
         private
 
-        attr_reader :reversed_order_expression, :nullable, :distinct, :order_direction
+        attr_reader :reversed_order_expression, :nullable, :distinct
 
         def calculate_reversed_order(order_expression)
           unless AREL_ORDER_CLASSES.has_key?(order_expression.class) # Arel can reverse simple orders

lib/gitlab/pagination/keyset/order.rb

@@ -139,6 +139,8 @@ module Gitlab
 
           verify_incoming_values!(values)
 
+          return use_composite_row_comparison(values) if composite_row_comparison_possible?
+
           where_values = []
 
           reversed_column_definitions = column_definitions.reverse
@@ -187,6 +189,28 @@ module Gitlab
 
         private
 
+        def composite_row_comparison_possible?
+          !column_definitions.one? &&
+            column_definitions.all?(&:not_nullable?) &&
+            column_definitions.map(&:order_direction).uniq.one? # all columns uses the same order direction
+        end
+
+        # composite row comparison works with NOT NULL columns and may use only one index scan given a proper index setup
+        # Example: (created_at, id) > ('2012-09-18 01:40:01+00', 15)
+        def use_composite_row_comparison(values)
+          columns = Arel::Nodes::Grouping.new(column_definitions.map(&:column_expression))
+          values = Arel::Nodes::Grouping.new(column_definitions.map do |column_definition|
+            value = values[column_definition.attribute_name]
+            Arel::Nodes.build_quoted(value, column_definition.column_expression)
+          end)
+
+          if column_definitions.first.ascending_order?
+            [columns.gt(values)]
+          else
+            [columns.lt(values)]
+          end
+        end
+
         # Adds extra columns to the SELECT clause
         def apply_custom_projections(scope)
           additional_projections = column_definitions.select(&:add_to_projections).map do |column_definition|

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

@@ -171,6 +171,12 @@ RSpec.describe Gitlab::Pagination::Keyset::Order do
       end
 
       it_behaves_like 'order examples'
+
+      it 'uses the row comparison method' do
+        sql = order.where_values_with_or_query({ year: 2010, month: 5, id: 1 }).to_sql
+
+        expect(sql).to eq('(("my_table"."year", "my_table"."month", "my_table"."id") > (2010, 5, 1))')
+      end
     end
 
     context 'when ordering by nullable columns and a distinct column' do