First, access or modification to the vPostgres databases of VMware appliances is not supported and should never take place without involving the GSS.
But vPostgres is just a normal PostgreSQL database and therefore all PostgreSQL commands for versions 9 and 10 are available.
messing about in the vCenter postgresql db is NOT supported by VMware and unless you have been directed by VMware to do so, you run the risk of being told your environment is unsupported.
if you want to learn PSQL I suggest you build a VM with it on and forget that vCenter is now running on Postgresql.
To further reinforce what the other vExperts have said, if you're asking this in context of poking around in the vCenter database, DO NOT do that. If you need to access the data within, you need to learn to use the API via one of many methods. Stay out of the vCenter database.