There's a case when Connection should be thread-safe: https://docs.oracle.com/javase/7/docs/api/java/sql/Statement.html#cancel() Wrong implementation of #cancel leads to issues like "wrong statement being cancelled".
For instance: https://github.com/pgjdbc/pgjdbc/pull/413 I'm not sure if #cancel has already been implemented in Calcite though. Pretty much the same issue applies to Statement#setQueryTimeout. Vladimir
