Custom fields and indexes
If your custom field type stores something else in the exp_channel_data table than what needs to be indexed, you can add Pro Search compatibility to your custom field type. Just add a public method third_party_search_index($field_data) to your field type class, and you’re good to go.
third_party_search_index is given one parameter: (string) $data, the field data currently in the database. Furthermore, all field settings are available through $this->settings, including $this->settings['entry_id'] for the current entry and $this->settings['pro_search_collection_id'] for the current collection id. The method should return a string that is used for the search index. The string will be sanitized by Pro Search and the field weight will be added automatically as well.
Example
public function third_party_search_index($data)
{
$query = $this->EE->db->select('real_content')
->from('custom_table')
->where('entry_id', $this->settings['entry_id'])
->where('other_key', $data)
->get();
return $query->row('real_content');
}