Time_precision and time_precision in Grafana queries?

Hello,
to save some space I’d like to set the time_precision to different values.

If I set influxdb> precision s, does this set the default write precision for all databases to “s”?
Can I set different defaults to different Databases and/or Measurements?
(I fill the InfluxDB with icinga2 which doesn’t offer afaik options for setting the precision)

When querying InfluxDB with Grafana and a non default precison, I have to use the time_precision in the query[1],
unfortunately I didn’t manage to implement this option in my query. How would I have to add it to e. g. the following query?

select value from cpu_metrics where host='myhost' and metric =~ /.*cpu.total.idle.*/ and time < now() - 30m

Thanks a lot for your help!

[1] https://github.com/influxdata/influxdb/issues/8050