diff --git a/test/helpers/functional_helpers.rb b/test/helpers/functional_helpers.rb
index 3d6dc9d34..d8df4e203 100644
--- a/test/helpers/functional_helpers.rb
+++ b/test/helpers/functional_helpers.rb
@@ -55,5 +55,23 @@ module FunctionalHelpers
     def json_response
       JSON.parse(@response.body)
     end
+
+    # JSON:API gives no meaning to the order of to-many linkage, so sort it before comparing responses
+    def normalize_linkage_order(json)
+      return json unless json.is_a?(Hash)
+
+      json.deep_dup.tap do |normalized|
+        resources = Array.wrap(normalized['data']) + Array.wrap(normalized['included'])
+        resources.each { |resource| sort_to_many_linkage(resource) }
+      end
+    end
+
+    private
+
+    def sort_to_many_linkage(resource)
+      (resource['relationships'] || {}).each_value do |relationship|
+        relationship['data'].sort_by! { |r| [r['type'], r['id']] } if relationship['data'].is_a?(Array)
+      end
+    end
   end
 end
\ No newline at end of file
diff --git a/test/helpers/functional_helpers_test.rb b/test/helpers/functional_helpers_test.rb
new file mode 100644
index 000000000..f892d6863
--- /dev/null
+++ b/test/helpers/functional_helpers_test.rb
@@ -0,0 +1,43 @@
+require File.expand_path('../../test_helper', __FILE__)
+
+class FunctionalHelpersTest < ActionController::TestCase
+
+  def test_normalize_linkage_order_sorts_to_many_linkage_in_data_and_included
+    json = {
+      'data' => [
+        { 'type' => 'books', 'id' => '1', 'relationships' => {
+          'book_comments' => { 'data' => [{ 'type' => 'book_comments', 'id' => '9' },
+                                          { 'type' => 'book_comments', 'id' => '2' }] } } }
+      ],
+      'included' => [
+        { 'type' => 'people', 'id' => '3', 'relationships' => {
+          'comments' => { 'data' => [{ 'type' => 'comments', 'id' => '5' },
+                                     { 'type' => 'comments', 'id' => '4' }] } } }
+      ]
+    }
+
+    normalized = normalize_linkage_order(json)
+
+    assert_equal %w[2 9], normalized['data'][0]['relationships']['book_comments']['data'].map { |r| r['id'] }
+    assert_equal %w[4 5], normalized['included'][0]['relationships']['comments']['data'].map { |r| r['id'] }
+  end
+
+  def test_normalize_linkage_order_leaves_everything_else_unchanged
+    assert_nil normalize_linkage_order(nil)
+
+    json = {
+      'data' => [
+        { 'type' => 'books', 'id' => '2', 'relationships' => {
+          'author' => { 'data' => { 'type' => 'people', 'id' => '7' } },
+          'tags' => { 'links' => { 'related' => '/books/2/tags' } } } },
+        { 'type' => 'books', 'id' => '1' }
+      ]
+    }
+
+    normalized = normalize_linkage_order(json)
+
+    assert_equal %w[2 1], normalized['data'].map { |r| r['id'] }
+    assert_equal({ 'type' => 'people', 'id' => '7' }, normalized['data'][0]['relationships']['author']['data'])
+    assert_equal({ 'links' => { 'related' => '/books/2/tags' } }, normalized['data'][0]['relationships']['tags'])
+  end
+end
diff --git a/test/integration/requests/request_test.rb b/test/integration/requests/request_test.rb
index b7895608c..bae5b9eaa 100644
--- a/test/integration/requests/request_test.rb
+++ b/test/integration/requests/request_test.rb
@@ -884,6 +884,19 @@ def test_pagination_related_resources_data_includes
     assert_equal 'This is comment 18 on book 1.', json_response['data'][9]['attributes']['body']
   end
 
+  def test_included_resource_inverse_linkage_with_table_statistics
+    # Once tables have statistics (autovacuum analyzes them mid-suite in CI), Postgres can plan
+    # SELECT DISTINCT as a HashAggregate, so the cached and uncached linkage come back in different orders.
+    ActiveRecord::Base.connection.execute('ANALYZE books')
+    ActiveRecord::Base.connection.execute('ANALYZE book_comments')
+    Api::V2::BookResource.paginator :offset
+    Api::V2::BookCommentResource.paginator :offset
+
+    assert_cacheable_jsonapi_get '/api/v2/books/1/book_comments?page[limit]=10&include=book'
+    linkage_ids = json_response['included'].first['relationships']['book_comments']['data'].map { |r| r['id'] }
+    assert_equal %w[51 53 55 57 59 61 63 65 67 69], linkage_ids.sort_by(&:to_i)
+  end
+
   def test_pagination_empty_results
     Api::V2::BookResource.paginator :offset
     Api::V2::BookCommentResource.paginator :offset
diff --git a/test/test_helper.rb b/test/test_helper.rb
index a02fff9e7..6108362a6 100644
--- a/test/test_helper.rb
+++ b/test/test_helper.rb
@@ -505,8 +505,8 @@ def assert_cacheable_jsonapi_get(url, cached_classes = :all)
     end
 
     assert_equal(
-      non_caching_response.pretty_inspect,
-      json_response.pretty_inspect,
+      normalize_linkage_order(non_caching_response).pretty_inspect,
+      normalize_linkage_order(json_response).pretty_inspect,
       "Cache warmup response must match normal response"
     )
 
@@ -515,8 +515,8 @@ def assert_cacheable_jsonapi_get(url, cached_classes = :all)
     end
 
     assert_equal(
-      non_caching_response.pretty_inspect,
-      json_response.pretty_inspect,
+      normalize_linkage_order(non_caching_response).pretty_inspect,
+      normalize_linkage_order(json_response).pretty_inspect,
       "Cached response must match normal response"
     )
     assert_equal 0, cached[:total][:misses], "Cached response must not cause any cache misses"
@@ -584,8 +584,8 @@ def assert_cacheable_get(action, **args)
           "Cache (mode: #{mode}) #{phase} response status must match normal response"
         )
         assert_equal(
-          non_caching_response.pretty_inspect,
-          json_response_sans_all_backtraces.pretty_inspect,
+          normalize_linkage_order(non_caching_response).pretty_inspect,
+          normalize_linkage_order(json_response_sans_all_backtraces).pretty_inspect,
           "Cache (mode: #{mode}) #{phase} response body must match normal response"
         )
         assert_operator(