As well as enabling the PGWire interface using Gateway and Gateway Network Parameters | Ignition User Manual, the username and password can be found in <ignition_dir>\data\var\com.inductiveautomation.historian\corehistorian\pg_credentials.json
The table structure is similar to the old SQL interface, but with a few improvements.
I’m still building example queries to get data with infill. It looks like types are respected, so Booleans and Strings are not interpolated.
Historian can now include Metadata (as JSON). This appears to include anything that is not default. It is lacking a bit for UDT structures in that this data is not included. This includes parameters and the UDT parents/relationships. I tried working around this with custom fields, but no luck yet.