Use case

Allow arbitrary WHERE condition (or no filtering at all) for SELECTs from system.zookeeper.

Conditions on prefix of path or a set of paths can be used to limit reads. Otherwise it will perform a full scan with recursive iteration using asynchronous requests.

Unrestricted reads can be disabled with a setting.

It can be used to collect statistics and analyze data distribution inside ZooKeeper. To perform complex queries on top of replicas and replication queue.

ZooKeeper is actually quite performant and can serve around 100 000 read requests per seconds on typical hardware. A dump of ~10 000 000 nodes can be created in minutes - this query speed is tolerable.

See also #22130.