Today I faced a performance problem caused by a bad cardinality estimation involving a CDB view in a 12.1.0.2 multitenant environment. While solving the problem I did a number of observations that I try to summarize in this blog post.
First of all, when checking the execution plan of a query already running for more than two hours, I noticed that, in the execution plan, neither the referenced CDB view nor one of its underlying objects were referenced. The following query (and its execution plan) executed while connect to the CDB illustrates (I also added the 12.2.0.1 output to show you the difference it that area):