Make sure (most) queries avoid table scans.

I've exempted inserts (they never scan in the first place), queries on `event_sequence` (at most one row), and the coalesce()s used for event replay (for now; these are obviously a performance risk area and need addressing).

Method:

```
find .sqlx -name 'query-*.json' -exec jq -r '"explain query plan " + .query + ";"' {} + > explain.sql
```

Then go query by query through the resulting file.

1 files changed, 62 insertions, 0 deletions

diff --git a/.sqlx/query-4623f989492e9eae6788ee9472f1138395b50cdfc9a28e9791e5484890f0201b.json b/.sqlx/query-4623f989492e9eae6788ee9472f1138395b50cdfc9a28e9791e5484890f0201b.json
new file mode 100644
index 0000000..bfab6d4
--- /dev/null
+++ b/.sqlx/query-4623f989492e9eae6788ee9472f1138395b50cdfc9a28e9791e5484890f0201b.json
@@ -0,0 +1,62 @@
+{
+  "db_name": "SQLite",
+  "query": "\n                select\n                    message.channel as \"channel: channel::Id\",\n                    message.sender as \"sender: login::Id\",\n                    id as \"id: Id\",\n                    message.body as \"body: Body\",\n                    message.sent_at as \"sent_at: DateTime\",\n                    message.sent_sequence as \"sent_sequence: Sequence\",\n                    deleted.deleted_at as \"deleted_at?: DateTime\",\n                    deleted.deleted_sequence as \"deleted_sequence?: Sequence\"\n                from message\n                left join message_deleted as deleted\n                    using (id)\n                where message.channel = $1\n                    and deleted.id is null\n            ",
+  "describe": {
+    "columns": [
+      {
+        "name": "channel: channel::Id",
+        "ordinal": 0,
+        "type_info": "Text"
+      },
+      {
+        "name": "sender: login::Id",
+        "ordinal": 1,
+        "type_info": "Text"
+      },
+      {
+        "name": "id: Id",
+        "ordinal": 2,
+        "type_info": "Text"
+      },
+      {
+        "name": "body: Body",
+        "ordinal": 3,
+        "type_info": "Text"
+      },
+      {
+        "name": "sent_at: DateTime",
+        "ordinal": 4,
+        "type_info": "Text"
+      },
+      {
+        "name": "sent_sequence: Sequence",
+        "ordinal": 5,
+        "type_info": "Integer"
+      },
+      {
+        "name": "deleted_at?: DateTime",
+        "ordinal": 6,
+        "type_info": "Text"
+      },
+      {
+        "name": "deleted_sequence?: Sequence",
+        "ordinal": 7,
+        "type_info": "Integer"
+      }
+    ],
+    "parameters": {
+      "Right": 1
+    },
+    "nullable": [
+      false,
+      false,
+      false,
+      true,
+      false,
+      false,
+      false,
+      false
+    ]
+  },
+  "hash": "4623f989492e9eae6788ee9472f1138395b50cdfc9a28e9791e5484890f0201b"
+}