When we're trying to create a new entry, we search for an existing entry
for the same URL and a similar owner. The index means we avoid having
to do a full scan of the table for this.
owner varchar(64) not null,
url text not null);
create index odin_shorturl_by_owner on odin_shorturl (owner);
owner varchar(64) not null,
url text not null);
create index odin_shorturl_by_owner on odin_shorturl (owner);
+create index odin_shorturl_by_url_owner on odin_shorturl (url, owner);