Select::preExecute

public Select::preExecute(SelectInterface $query = NULL)

public function preExecute(SelectInterface $query = NULL) {
  // If no query object is passed in, use $this.
  if (!isset($query)) {
    $query = $this;
  }

  // Only execute this once.
  if ($query->isPrepared()) {
    return TRUE;
  }

  // Modules may alter all queries or only those having a particular tag.
  if (isset($this->alterTags)) {
    $hooks = array('query');
    foreach ($this->alterTags as $tag => $value) {
      $hooks[] = 'query_' . $tag;
    }
    \Drupal::moduleHandler()->alter($hooks, $query);
  }

  $this->prepared = TRUE;

  // Now also prepare any sub-queries.
  foreach ($this->tables as $table) {
    if ($table['table'] instanceof SelectInterface) {
      $table['table']->preExecute();
    }
  }

  foreach ($this->union as $union) {
    $union['query']->preExecute();
  }

  return $this->prepared;
}