Random spikes in usage (CockroachCloud Serverless)

Question

I recently set up a free CockroachDB Serverless cluster on CockroachCloud. It's been really great so far, but sometimes there are random spikes in Request Units even though the amount of SQL statements doesn't increase at all. Here's a screenshot of the two graphs in the cluster management page, it illustrates pretty well what I mean. I would really appreciate some help on how I could eliminate these spikes because CockroachCloud has some limits on free usage. That being said, I'm still fairly new to CockroachDB, so I might be missing something obvious.

Answer 1

You are likely performing enough mutations on your data to trigger automatic statistics collection as a background process. By default when 20% or more rows are modified in a table, CockroachDB will trigger a statistics refresh. The statistics are used by the optimizer to create more efficient query plans.

Your SQL Statements graph indicates that almost all your operations are inserts. That many inserts is almost certainly triggering stats collection. While you can turn off stats collection, the optimizer will then be using stale data to calculate query plans, potentially causing performance problems.

The occasional spikes in your Request Unit graph are above the 100 RUs per second baseline, but the rest of the time you are well below 100 RUs per second. That means you are accumulating RUs most of the time, and that (plus the initial 10 million RU allocation) should cover the bursts.

I added a FAQ entry to the Serverless docs covering this.