diff --git a/core/modules/system/tests/modules/database_test/database_test.install b/core/modules/system/tests/modules/database_test/database_test.install
index effd261994db612edc7dcb2e96b7d8fb9e064036..47ed774a590a1ecabe07297dbbfec31c75d009e5 100644
--- a/core/modules/system/tests/modules/database_test/database_test.install
+++ b/core/modules/system/tests/modules/database_test/database_test.install
@@ -320,7 +320,9 @@ function database_test_schema() {
     'fields' => [
       'id' => [
         'description' => 'Simple unique ID.',
-        'type' => 'int',
+        // Using a serial as an ID properly tests
+        // \Drupal\Core\Database\Driver\pgsql\Upsert.
+        'type' => 'serial',
         'not null' => TRUE,
       ],
       'update' => [
diff --git a/core/tests/Drupal/KernelTests/Core/Database/UpsertTest.php b/core/tests/Drupal/KernelTests/Core/Database/UpsertTest.php
index 30474c541d98f2bae0e377d1a80cf69cc366a139..f46fd41e66b23a79c73bcfe5b63d10d5402cbee4 100644
--- a/core/tests/Drupal/KernelTests/Core/Database/UpsertTest.php
+++ b/core/tests/Drupal/KernelTests/Core/Database/UpsertTest.php
@@ -68,7 +68,8 @@ public function testUpsertWithKeywords() {
 
     // Add a new row.
     $upsert->values([
-      'id' => 2,
+      // Test a non sequence ID for better testing of the default return value.
+      'id' => 3,
       'update' => 'Update value 2',
     ]);
 
@@ -80,6 +81,10 @@ public function testUpsertWithKeywords() {
 
     $result = $upsert->execute();
     $this->assertIsInt($result);
+    // The upsert returns the number of rows affected. For MySQL the return
+    // value is 3 because the affected-rows value per row is 1 if the row is
+    // inserted as a new row, 2 if an existing row is updated. See
+    // https://dev.mysql.com/doc/c-api/8.0/en/mysql-affected-rows.html.
     $this->assertGreaterThanOrEqual(2, $result, 'The result of the upsert operation should report that at least two rows were affected.');
 
     $num_records_after = $this->connection->query('SELECT COUNT(*) FROM {select}')->fetchField();
@@ -88,8 +93,18 @@ public function testUpsertWithKeywords() {
     $record = $this->connection->query('SELECT * FROM {select} WHERE [id] = :id', [':id' => 1])->fetch();
     $this->assertEquals('Update value 1 updated', $record->update);
 
-    $record = $this->connection->query('SELECT * FROM {select} WHERE [id] = :id', [':id' => 2])->fetch();
+    $record = $this->connection->query('SELECT * FROM {select} WHERE [id] = :id', [':id' => 3])->fetch();
     $this->assertEquals('Update value 2', $record->update);
+
+    // An upsert should be re-usable.
+    $upsert->values([
+      'id' => 4,
+      'update' => 'Another value',
+    ]);
+    $return_value = $upsert->execute();
+    $this->assertSame(1, $return_value);
+    $record = $this->connection->query('SELECT * FROM {select} WHERE [id] = :id', [':id' => 4])->fetch();
+    $this->assertEquals('Another value', $record->update);
   }
 
   /**