materio-base-d7/sites/all/modules/examples/field_permission_example/field_permission_example.install

<?php
/**
 * @file
 * Install function for the field_permission_example module.
 */

/**
 * Implements hook_field_schema().
 *
 * Defines the database schema of the field, using the format used by the
 * Schema API.
 *
 * We only want a simple text field, similar to the body of a node.
 *
 * Note that this field has only a normal text (which translates to
 * 16k of text in MySQL), and so therefore doesn't have an index.
 * More complex schema would probably have at least one indexed
 * column.
 *
 * @see http://drupal.org/node/146939
 * @see schemaapi
 * @see hook_field_schema()
 * @ingroup field_permission_example
 */
function field_permission_example_field_schema($field) {
  $columns = array(
    'notes' => array('type' => 'text', 'size' => 'normal', 'not null' => FALSE),
  );
  return array(
    'columns' => $columns,
  );
}