Sam 37088c4221 PERF: index on topic_id for user_histories table
We query this table when getting composer messages with the queries
such as:

```
SELECT  1 AS one FROM "user_histories"
WHERE "user_histories"."target_user_id" = 1 AND
 "user_histories"."action" = 9 AND
 "user_histories"."topic_id" = 105794
LIMIT 1
```

This index ensures this query remains very quick, regardless of user
history size.
2019-01-06 15:14:41 +11:00
..
2018-11-21 00:45:16 +01:00