0039_unique_series_references migration needs optimization

[Konstantin Ryabitsev]

Hello:

I may be venting some frustration, but I'm now 4 hours into a 
2.1 -> 2.2 upgrade. The following query from 0039 has now been running 
over 3 hours:

      COMMAND: Query
         TIME: 10921
        STATE: Sending data
         INFO: DELETE a FROM patchwork_seriesreference a
            INNER JOIN patchwork_seriesreference b
            WHERE
              a.id < b.id
              AND a.project_id = b.project_id
              AND a.msgid = b.msgid

That table has 220,000 rows, but it still seems strange that it's taking 
so long to execute. As there is no way to know if it's about to be 
finished or will take another few hours to run, I'm not willing to kill 
it yet.

Would adding an index across (id, project_id, msgid) make this query 
faster to run?

-K