FAQ

Frequently Asked Questions

```
 File "/home/ubuntu/ottertune/server/website/website/parser/base.py", line 302, in calculate_change_in_metrics
 assert adj_val >= 0, '{} wrong metric type '.format(met_name)
 AssertionError: pg_stat_user_tables.n_mod_since_analyze wrong metric type
```
This is because the value in metric_after.json is smaller than that in metric_before.json, the change is smaller than 0. If the metric type is MetricType.COUNTER, we calculate the change and require it is positive. Otherwise we set the metric as the value in metric_after.json. Thus, you can fix it by changing the metric type as 2 (MetricType.INFO) or 3 (MetricType.STATISTICS) in the metric fixture.
How to tune a new database ?

A brief answer is to add a collector in the client side, and a parser in the server side. OtterTune also needs knob and metric fixture of the target database. You do not need to collect all the information of knobs/metrics, start with some important ones. This PR is an example to support SAP HANA. (https://github.com/cmu-db/ottertune/pull/194)
How to define a new target objective besides throughput and latency ?

You can define your own function as a target objective, you should collect its information in the client side. This PR has some description. (https://github.com/cmu-db/ottertune/pull/182)