1 files changed, 28 insertions, 5 deletions

diff --git a/lib/gitlab/pagination/keyset/order.rb b/lib/gitlab/pagination/keyset/order.rb
index e596e1bac9d..cef3a7b291a 100644
--- a/lib/gitlab/pagination/keyset/order.rb
+++ b/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
 
@@ -170,6 +183,8 @@ module Gitlab
           self.class.build(column_definitions.map(&:reverse))
         end
 
+        alias_method :to_sql, :to_s
+
         private
 
         # Adds extra columns to the SELECT clause
@@ -210,11 +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)