If you are installing trail version of CDP , It will use embedded Postgre for installation.
Installing a Trial Cluster
In this procedure, Cloudera Manager automates the installation of the Oracle JDK, Cloudera Manager Server, embedded PostgreSQL database, Cloudera Manager Agent, Runtime, and managed service software on cluster hosts. Cloudera Manager also configures databases for the Cloudera Manager Server and Hive Metastore and optionally for Cloudera Management Service roles."
You can check below link for detail explanation.