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
@@ -61,7 +61,7 @@
     $args = func_get_args();
     $info = $this->getPrefixInfo($identifier);
     $args[0] = $info['table'];
-    $identifierName = implode('_', $args);
+    $identifierName = implode('__', $args);
 
     // Retrieve the max identifier length which is usually 63 characters
     // but can be altered before PostgreSQL is compiled so we need to check.