Add further common Postgres Unix socket URL examples

[malteneuss]

Add more and shorter URL syntax examples when no password is needed for the Unix socket connection.

I had struggled to understand shortened socket connections strings in Postgres and would have liked these examples to make it obvious that parts are simply left out.

Feel free to close or edit as you please, and thanks for this great tool.

edit: shameless bump @amacneil

[amacneil]

Does this work? I would have expected needing at least postgres://username@/database_name?socket=/var/run/postgresql (with @, to differentiate it from a hostname)

[malteneuss]

Yes, i tested it with Postgres 14.9.

[dossy]

@amacneil This only works with a local connection with the peer authentication method, because in that specific mode, Postgres isn't using the hostname or username from the connection string, as it's connecting locally (so hostname is irrelevant) and peer auth relies on Postgres getting the OS username, so the username from the connection string is irrelevant.

In other words, this works:

postgres@e8f183dbe0dd:/tmp$ ./dbmate -u postgres:///dbmate_test status
[ ] 20231115033218_test.sql

Applied: 0
Pending: 1

Notice that I'm doing this as user postgres, which happens to be a valid database user. If I try this as user root that isn't a valid user in this Postgres database, we get this error:

root@e8f183dbe0dd:/tmp# ./dbmate -u postgres:///dbmate_test status
Error: pq: role "root" does not exist

[amacneil]

Huh, this is also surprising to me. I guess it's postgres-specific behavior. It's not something dbmate does.

[malteneuss]

Same, i tested it with Postgres 14.9.

[dossy]

It actually has nothing to do with the username and database name being identical. When relying on Postgres peer authentication method, specifying the username and hostname in the connection string are unnecessary:

Check this out:

postgres@e8f183dbe0dd:/tmp$ ./dbmate -u postgres://hostnameIsIgnoredWhenSocketIsUsed/dbmate_test?socket=/var/run/postgresql status
[ ] 20231115033218_test.sql

Applied: 0
Pending: 1

postgres@e8f183dbe0dd:/tmp$ ./dbmate -u postgres:///dbmate_test?socket=/var/run/postgresql status
[ ] 20231115033218_test.sql

Applied: 0
Pending: 1

I think adding these shorthand Postgres database URL examples to the documenation might be handy, but the current PR's explanation for why they work in the README are not accurate.