What does Type "ref|filter" mean in mariaDB Explain Output

Question

Trying to troubleshoot a MariaDB query. We did an explain select on the select. A couple of the rows in the output have value ref|filter for the type column.

What does this mean? The MariaDB docs mention ref but make no mention of filter or ref|filter as a possible value for this column.

Using MariaDB 10.5

Answer 1

This is rowid filtering where the filter refers the usage of the second index.