diff --git a/core/lib/Drupal/Core/Database/Query/Select.php b/core/lib/Drupal/Core/Database/Query/Select.php
index cb131a9..dde6808 100644
--- a/core/lib/Drupal/Core/Database/Query/Select.php
+++ b/core/lib/Drupal/Core/Database/Query/Select.php
@@ -710,7 +710,11 @@ public function __toString() {
         $table_string = '(' . (string) $subquery . ')';
       }
       else {
-        $table_string = '{' . $this->connection->escapeTable($table['table']) . '}';
+        $table_string = $this->connection->escapeTable($table['table']);
+        // Do not attempt prefixing cross database / schema queries.
+        if (strpos($table_string, '.') === FALSE) {
+          $table_string = '{' . $table_string . '}';
+        }
       }
 
       // Don't use the AS keyword for table aliases, as some
diff --git a/core/lib/Drupal/Core/Database/Query/SelectInterface.php b/core/lib/Drupal/Core/Database/Query/SelectInterface.php
index 8e1c757..db66aef 100644
--- a/core/lib/Drupal/Core/Database/Query/SelectInterface.php
+++ b/core/lib/Drupal/Core/Database/Query/SelectInterface.php
@@ -216,7 +216,10 @@ public function addExpression($expression, $alias = NULL, $arguments = array());
    * This method is a convenience method for innerJoin().
    *
    * @param $table
-   *   The table against which to join.
+   *   The table against which to join. May be a string or another SelectQuery
+   *   object. If a query object is passed, it will be used as a subselect.
+   *   Unless the table name starts with the database / schema name and a dot
+   *   it will be prefixed.
    * @param $alias
    *   The alias for the table. In most cases this should be the first letter
    *   of the table, or the first letter of each "word" in the table.
@@ -239,7 +242,10 @@ public function join($table, $alias = NULL, $condition = NULL, $arguments = arra
    * Inner Join against another table in the database.
    *
    * @param $table
-   *   The table against which to join.
+   *   The table against which to join. May be a string or another SelectQuery
+   *   object. If a query object is passed, it will be used as a subselect.
+   *   Unless the table name starts with the database / schema name and a dot
+   *   it will be prefixed.
    * @param $alias
    *   The alias for the table. In most cases this should be the first letter
    *   of the table, or the first letter of each "word" in the table.
@@ -262,7 +268,10 @@ public function innerJoin($table, $alias = NULL, $condition = NULL, $arguments =
    * Left Outer Join against another table in the database.
    *
    * @param $table
-   *   The table against which to join.
+   *   The table against which to join. May be a string or another SelectQuery
+   *   object. If a query object is passed, it will be used as a subselect.
+   *   Unless the table name starts with the database / schema name and a dot
+   *   it will be prefixed.
    * @param $alias
    *   The alias for the table. In most cases this should be the first letter
    *   of the table, or the first letter of each "word" in the table.
@@ -285,7 +294,10 @@ public function leftJoin($table, $alias = NULL, $condition = NULL, $arguments =
    * Right Outer Join against another table in the database.
    *
    * @param $table
-   *   The table against which to join.
+   *   The table against which to join. May be a string or another SelectQuery
+   *   object. If a query object is passed, it will be used as a subselect.
+   *   Unless the table name starts with the database / schema name and a dot
+   *   it will be prefixed.
    * @param $alias
    *   The alias for the table. In most cases this should be the first letter
    *   of the table, or the first letter of each "word" in the table.
@@ -316,6 +328,8 @@ public function rightJoin($table, $alias = NULL, $condition = NULL, $arguments =
    * @param $table
    *   The table against which to join. May be a string or another SelectQuery
    *   object. If a query object is passed, it will be used as a subselect.
+   *   Unless the table name starts with the database / schema name and a dot
+   *   it will be prefixed.
    * @param $alias
    *   The alias for the table. In most cases this should be the first letter
    *   of the table, or the first letter of each "word" in the table. If omitted,