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, 7 insertions, 9 deletions

diff --git a/src/message/repo.rs b/src/message/repo.rs
index 4cfefec..c8ceceb 100644
--- a/src/message/repo.rs
+++ b/src/message/repo.rs
@@ -79,8 +79,8 @@ impl<'c> Messages<'c> {
                     message.body as "body: Body",
                     message.sent_at as "sent_at: DateTime",
                     message.sent_sequence as "sent_sequence: Sequence",
-                    deleted.deleted_at as "deleted_at: DateTime",
-                    deleted.deleted_sequence as "deleted_sequence: Sequence"
+                    deleted.deleted_at as "deleted_at?: DateTime",
+                    deleted.deleted_sequence as "deleted_sequence?: Sequence"
                 from message
                 left join message_deleted as deleted
                     using (id)
@@ -186,33 +186,31 @@ impl<'c> Messages<'c> {
     ) -> Result<History, sqlx::Error> {
         let id = message.id();
 
-        sqlx::query_scalar!(
+        sqlx::query!(
             r#"
                 insert into message_deleted (id, deleted_at, deleted_sequence)
                 values ($1, $2, $3)
-                returning 1 as "deleted: bool"
             "#,
             id,
             deleted.at,
             deleted.sequence,
         )
-        .fetch_one(&mut *self.0)
+        .execute(&mut *self.0)
         .await?;
 
         // Small social responsibility hack here: when a message is deleted, its body is
         // retconned to have been the empty string. Someone reading the event stream
         // afterwards, or looking at messages in the channel, cannot retrieve the
         // "deleted" message by ignoring the deletion event.
-        sqlx::query_scalar!(
+        sqlx::query!(
             r#"
                 update message
-                set body = ""
+                set body = ''
                 where id = $1
-                returning 1 as "blanked: bool"
             "#,
             id,
         )
-        .fetch_one(&mut *self.0)
+        .execute(&mut *self.0)
         .await?;
 
         let message = self.by_id(id).await?;