Add an index to the repository creation timestamp

Created by: flying-robot

The repositories GraphQL endpoint supports sorting via either REPOSITORY_NAME or REPOSITORY_CREATED_AT. Using the timestamp doesn't hit any indexes, resulting in a much more expensive query plan:

localhost sg@sg=# EXPLAIN SELECT * FROM repo ORDER BY name DESC LIMIT 5;
┌─────────────────────────────────────────────────────────────────────────────────────────────────────────┐
│                                               QUERY PLAN                                                │
├─────────────────────────────────────────────────────────────────────────────────────────────────────────┤
│ Limit  (cost=0.56..1.28 rows=5 width=530)                                                               │
│   ->  Index Scan Backward using repo_name_unique on repo  (cost=0.56..517029.02 rows=3576655 width=530) │
└─────────────────────────────────────────────────────────────────────────────────────────────────────────┘
(2 rows)

localhost sg@sg=# EXPLAIN SELECT * FROM repo ORDER BY created_at DESC LIMIT 5;
┌───────────────────────────────────────────────────────────────────────────────────────────┐
│                                        QUERY PLAN                                         │
├───────────────────────────────────────────────────────────────────────────────────────────┤
│ Limit  (cost=394376.44..394377.04 rows=5 width=530)                                       │
│   ->  Gather Merge  (cost=394376.44..822626.06 rows=3576656 width=530)                    │
│         Workers Planned: 4                                                                │
│         ->  Sort  (cost=393376.38..395611.79 rows=894164 width=530)                       │
│               Sort Key: created_at DESC                                                   │
│               ->  Parallel Seq Scan on repo  (cost=0.00..378524.64 rows=894164 width=530) │
└───────────────────────────────────────────────────────────────────────────────────────────┘
(6 rows)

The difference in wall-clock time is very noticeable as a result:

localhost sg@sg=# SELECT id FROM repo ORDER BY name DESC LIMIT 5;
┌──────────┐
│    id    │
├──────────┤
│ 41271131 │
│ 41270656 │
│ 41618978 │
│ 41164256 │
│ 42024836 │
└──────────┘
(5 rows)
Time: 60.597 ms

localhost sg@sg=# SELECT id FROM repo ORDER BY created_at DESC LIMIT 5;
┌──────────┐
│    id    │
├──────────┤
│ 42090777 │
│ 42090776 │
│ 42090775 │
│ 42090774 │
│ 42090773 │
└──────────┘
(5 rows)
Time: 701.684 ms