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, 56 insertions, 0 deletions

diff --git a/.sqlx/query-a4b34593fdf71bb911beb850cfa88adb346b1770e32785166bd43cb853143a7f.json b/.sqlx/query-a4b34593fdf71bb911beb850cfa88adb346b1770e32785166bd43cb853143a7f.json
new file mode 100644
index 0000000..e886759
--- /dev/null
+++ b/.sqlx/query-a4b34593fdf71bb911beb850cfa88adb346b1770e32785166bd43cb853143a7f.json
@@ -0,0 +1,56 @@
+{
+  "db_name": "SQLite",
+  "query": "\n                select\n                    channel.id as \"id: Id\",\n                    name.display_name as \"display_name?: String\",\n                    name.canonical_name as \"canonical_name?: String\",\n                    channel.created_at as \"created_at: DateTime\",\n                    channel.created_sequence as \"created_sequence: Sequence\",\n                    deleted.deleted_at as \"deleted_at?: DateTime\",\n                    deleted.deleted_sequence as \"deleted_sequence?: Sequence\"\n                from channel\n                left join channel_name as name\n                    using (id)\n                left join channel_deleted as deleted\n                    using (id)\n                left join message\n                    on channel.id = message.channel\n                where channel.created_at < $1\n                    and message.id is null\n                    and deleted.id is null\n            ",
+  "describe": {
+    "columns": [
+      {
+        "name": "id: Id",
+        "ordinal": 0,
+        "type_info": "Text"
+      },
+      {
+        "name": "display_name?: String",
+        "ordinal": 1,
+        "type_info": "Null"
+      },
+      {
+        "name": "canonical_name?: String",
+        "ordinal": 2,
+        "type_info": "Null"
+      },
+      {
+        "name": "created_at: DateTime",
+        "ordinal": 3,
+        "type_info": "Text"
+      },
+      {
+        "name": "created_sequence: Sequence",
+        "ordinal": 4,
+        "type_info": "Integer"
+      },
+      {
+        "name": "deleted_at?: DateTime",
+        "ordinal": 5,
+        "type_info": "Text"
+      },
+      {
+        "name": "deleted_sequence?: Sequence",
+        "ordinal": 6,
+        "type_info": "Integer"
+      }
+    ],
+    "parameters": {
+      "Right": 1
+    },
+    "nullable": [
+      false,
+      false,
+      false,
+      false,
+      false,
+      false,
+      false
+    ]
+  },
+  "hash": "a4b34593fdf71bb911beb850cfa88adb346b1770e32785166bd43cb853143a7f"
+}