See parent tasks. These have been approved in T404995#11270829 and T406146#11281383.

ALTERs to run:

ALTER TABLE ce_event_contributions ADD cec_user_name VARBINARY(255) DEFAULT NULL AFTER cec_user_id;
CREATE INDEX cec_user_id_name ON ce_event_contributions (cec_user_id, cec_user_name);
CREATE UNIQUE INDEX cec_wiki_revision ON ce_event_contributions (cec_wiki, cec_revision_id);

1. Where to run those changes:
   1. x1.testwiki
   2. x1.test2wiki
   3. x1.officewiki
   4. x1.wikishared
2. When to run those changes: Next week, once everything is on wmf.24 (due to r1195761)
3. If the schema change is backwards compatible: Yes (the new column is default null and all usages are behind a feature flag that's turned off in production)
4. If the schema change has been tested already on some of the test/beta wikis: Tested locally
5. if the data should be made available on the labs replicas and/or dumps: The table is in x1 and therefore not replicated.
6. Is this schema change blocking a feature rollout? Yes