how do I empty residing traces from signoz? seems like I have some memory leak where it is always growing even with retention only 1h

The minimum time required for ClickHouse to trigger the merge process is 4 hours but that's not guaranteed. Tables in SigNoz drop data in full parts i.e even if there is one record data part in ttl limit it won't be dropped. If you are not seeing any difference after a day then there is a issue otherwise this is expected.