Checks if an index exists in the given table.
$table: The name of the table in drupal (no prefixing).
$name: The name of the index in drupal (no prefixing).
TRUE if the given index exists, otherwise FALSE.
Overrides DatabaseSchema::indexExists
public function indexExists($table, $name) {
  // Details http://www.postgresql.org/docs/8.3/interactive/view-pg-indexes.html
  $index_name = '{' . $table . '}_' . $name . '_idx';
  return (bool) $this->connection
    ->query("SELECT 1 FROM pg_indexes WHERE indexname = '{$index_name}'")
    ->fetchField();
}