Take action when the search index is going to be rebuilt.
Modules that use hook_update_index() should update their indexing bookkeeping so that it starts from scratch the next time hook_update_index() is called.
Note: this list is generated by pattern matching, so it may include some functions that are not actually implementations of this hook.
function hook_search_reset() {
db_update('search_dataset')
->fields(array(
'reindex' => REQUEST_TIME,
))
->condition('type', 'node')
->execute();
}