@amey84 Yes. Although yum install still provides the bundled postgres, you can choose to install it or another database separately. During ambari-server setup you choose Y here:
Enter advanced database configuration [y/n] (n)? y
The following links will be helpful here for more info about ambari + postgres:
https://docs.cloudera.com/HDPDocuments/Ambari-2.6.1.5/bk_ambari-administration/content/using_ambari_...
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Thanks,
Steven