field_test_schema_alter.install

<?php
/**
 * @file
 * Install, update and uninstall functions for the field_test_schema_alter
 * module.
 */

/**
 * Implements hook_field_schema_alter().
 */
function field_test_schema_alter_field_schema_alter(&$schema, $field) {
  if ($field['type'] == 'test_field') {
    // Alter the field type.
    $schema['columns']['value']['type'] = 'float';
    // Add an additional column of data.
    $schema['columns']['additional_column'] = array(
      'description' => "Additional column added to image field table.",
      'type' => 'varchar',
      'length' => 128,
      'not null' => FALSE,
    );
  }
}