Should add some indexs to speed up

[kunzhipeng]

It's very slow when pull tasks via next(), finally i find there is no any index. After i added following indexs, it's fast now. collection.ensure_index([('priority', ASCENDING)]) collection.ensure_index([('locked_by', ASCENDING), ('locked_at', ASCENDING), ('attempts', ASCENDING)]) collection.ensure_index([('_id', ASCENDING), ('locked_by', ASCENDING)])