There are two indices that use the "nature" column in tracker_changeset_value_artifactlink DB table. Both set the index prefix length to 10 characters.

It turns out that the index was not being used at all, leading to a full table scan on SQL queries like the following:

SELECT DISTINCT nature AS shortname
FROM tracker_changeset_value_artifactlink
WHERE nature IS NOT NULL

This query is run to get the list of used artifact link types.

Follow-up of request #10127 and story #8833.