diff -u b/core/lib/Drupal/Core/Database/Driver/pgsql/Schema.php b/core/lib/Drupal/Core/Database/Driver/pgsql/Schema.php
--- b/core/lib/Drupal/Core/Database/Driver/pgsql/Schema.php
+++ b/core/lib/Drupal/Core/Database/Driver/pgsql/Schema.php
@@ -10,6 +10,7 @@
 use Drupal\Component\Utility\Unicode;
 use Drupal\Core\Database\Database;
 use Drupal\Core\Database\Query\Condition;
+use Drupal\Core\Database\SchemaIndexNotInSpecException;
 use Drupal\Core\Database\SchemaObjectExistsException;
 use Drupal\Core\Database\SchemaObjectDoesNotExistException;
 use Drupal\Core\Database\Schema as DatabaseSchema;
only in patch2:
unchanged:
--- /dev/null
+++ b/core/lib/Drupal/Core/Database/SchemaIndexNotInSpecException.php
@@ -0,0 +1,13 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Core\Database\SchemaIndexNotInSpecException
+ */
+
+namespace Drupal\Core\Database;
+
+/**
+ * Exception thrown if an index is not defined in a table schema specification.
+ */
+class SchemaIndexNotInSpecException extends SchemaException implements DatabaseException { }
only in patch2:
unchanged:
--- a/core/modules/system/src/Tests/Database/SchemaTest.php
+++ b/core/modules/system/src/Tests/Database/SchemaTest.php
@@ -295,6 +295,11 @@ function testIndexLength() {
     );
     db_create_table('test_table_index_length', $table_specification);
 
+    // Add a separate index.
+    $table_specification_new = $table_specification;
+    $table_specification_new['indexes']['test_separate'] = array('test_field_text', 200);
+    Database::getConnection()->schema()->addNormalizedIndex('test_table_index_length', 'test_separate', $table_specification_new);
+
     // Get index information.
     $results = db_query('SHOW INDEX FROM {test_table_index_length}');
     $expected_lengths = array(
@@ -316,6 +321,9 @@ function testIndexLength() {
         'test_field_string_ascii_long' => 200,
         'test_field_string_short' => NULL,
       ),
+      'test_separate' => array(
+        'test_field_text' => 191,
+      ),
     );
 
     // Count the number of columns defined in the indexes.