A source index a GROUP BY or JOIN target needs and lacks, as
Trellis.status/2 reports it: the qualified table, the columns the
index should lead with, the part of the definition that needs it (role:
:group_by for the GROUP BY columns, :join_from or :join_joined for
a JOIN's ON columns on its from or joined side), whether the transform
opted out with WITHOUT SOURCE INDEX (opted_out), and a create index
statement to run (fix).
Trellis doesn't create indexes on your tables.