Setting

require_fully_parsed=false

seems to be a workaround.

require_fully_parsed=false is a workaround, although it does mean that you may be able to bypass the masking. E.g. you use a function for hiding the column name that should be masked in a statement that MaxScale fails to parse.

Just for the record, the problem is in the parser, based upon sqlite3, that MaxScale uses. Sqlite3 does not know the EXPLAIN statement as it is in MariaDB but it is recognized merely based on the presence of the EXPLAIN keyword. That's sufficient for routing purposes because the EXPLAIN can simply be sent to any slave as it does not modify anything. However, that causes the masking filter to reject the statement since it wasn't parsed. The proper fix would be to extend the parser, but a straightforward change of the grammar causes numerous parsing conflicts, so adding support for EXPLAIN is much harder than what one might assume. So the feasible fix is for the masking filter to pass a statement directly through if it is EXPLAIN or ANALYZE irrespective of whether the whole statement could be parsed or not.

From 22.08.0 onward all EXPLAIN, DESCRIBE and ANALYZE statements will be passed through, as they do not return any user data.