Use "=" instead of IN

The subquery should only return a single row so there is no need to
use IN. Using "=" avoids a situation where the planner can sidestep
the LIMIT in the subquery.
collectiveidea#169

lib/delayed/backend/active_record.rb

@@ -128,7 +128,7 @@ def self.reserve_with_scope_using_optimized_postgres(ready_scope, worker, now)
           # use 'FOR UPDATE' and we would have many locking conflicts
           quoted_name = connection.quote_table_name(table_name)
           subquery    = ready_scope.limit(1).lock(true).select("id").to_sql
-          sql         = "UPDATE #{quoted_name} SET locked_at = ?, locked_by = ? WHERE id IN (#{subquery}) RETURNING *"
+          sql         = "UPDATE #{quoted_name} SET locked_at = ?, locked_by = ? WHERE id = (#{subquery}) RETURNING *"
           reserved    = find_by_sql([sql, now, worker.name])
           reserved[0]
         end