Getting the list of artifact links types (natures) can be slow on big databases because of prefix index (nature(10)). This affect two queries:

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

as well as

SELECT DISTINCT nature
                  FROM tracker_changeset_value_artifactlink
                  JOIN tracker_changeset_value ON (tracker_changeset_value_artifactlink.changeset_value_id = tracker_changeset_value.id)
                  JOIN tracker_changeset ON (tracker_changeset_value.changeset_id = tracker_changeset.id)
                  JOIN tracker_artifact ON (tracker_changeset.artifact_id = tracker_artifact.id)
                  JOIN tracker ON (tracker_artifact.tracker_id = tracker.id)
                 WHERE tracker.group_id = '879'
                 ORDER BY nature ASC;

The index should be on full field otherwise, due to DISTINCT the index cannot be used and generated full scan of the table.