Issue #2226501 by drunken monkey: Add execute() to the SelectInterface.

core/lib/Drupal/Core/Database/Query/SelectInterface.php

@@ -478,6 +478,14 @@ public function isPrepared();
    */
   public function preExecute(SelectInterface $query = NULL);
 
+  /**
+   * Runs the query against the database.
+   *
+   * @return \Drupal\Core\Database\StatementInterface|null
+   *   A prepared statement, or NULL if the query is not valid.
+   */
+  public function execute();
+
   /**
    * Helper function to build most common HAVING conditional clauses.
    *