It's a bug fix release of 4.0.9.
If you have already upgraded to 4.0.9, please upgrade to 4.1.0. And re-create your all indexes. You can re-create your indexes by removing and re-creating your indexes such as:
column_remove TABLE INDEX_COLUMN
column_create TABLE INDEX_COLUMN COLUMN_INDEX|...
See also
Note
If you didn't change your indexes by Groonga 4.0.9, you don't need to re-create your indexes. "change" means "add", "remove" and "update". If you use your indexes only for "search" by Groonga 4.0.9, you don't need to re-create your indexes.
Caution
Use 4.1.0 instead of 4.0.9. 4.0.9 has a serious bug that may break index.