Adding sorts for long text fields in Views

custom.module

<?php

/**
 * Implements hook_field_views_data_alter().
 *
 * Views disables sorting on any field columns that have a schema type of 'text'
 * even though it's valid to sort on those fields. Force those field columns to
 * have a sort available in Views.
 *
 * CAUTION: Sorting on these fields may cause performance issues with queries.
 */
function custom_field_views_data_alter(&$data, $field, $module) {
  foreach ($field['columns'] as $column_name => $column) {
    if ($column['type'] == 'text') {
      // field_views_data() only processes fields using sql storage.
      $storage_column_name = _field_sql_storage_columnname($field['field_name'], $column_name);
      foreach ($data as $table => &$field_data) {
        if (empty($field_data[$storage_column_name]['sort'])) {
          $field_data[$storage_column_name]['sort'] = array(
            'field' => $storage_column_name,
            'table' => $table,
            'handler' => 'views_handler_sort',
            'additional fields' => $field_data[$storage_column_name]['filter']['additional fields'],
            'field_name' => $field['field_name'],
          );
        }
      }
    }
  }
}