[Cockroach Labs engineer here, I worked on statistics]

All table statistics are stored in a system table (system.table_statistics). CREATE STATISTICS scans the table, calculating statistics on a column, and adds a new row to system.table_statistics.

We still have a ways to go in making statistics automatic (right now it requires a manual call to CREATE STATISTICS) and more useful to the optimizer (which currently only uses basic stats). We're working on that right now for the 2.2 release.

What if the tables were partitioned, do you calculate local statistics or you aggregate the stats and expose it to the optimizer?

Are you planning to support multivariate statistics?

We plan to support cardinality estimations for multiple columns. Other than that there are no plans for multivariate stats in the near future.