Обсуждение: [NOVICE] Why psql connection assumes default database name as the username

On Thu, Mar 23, 2017 at 03:57:58PM +1100, Neha Khatri wrote:
> For psql connection, the default database name is assumed to be operating
> system user name. Is it a common practice to have a database name same as
> an OS username. Or would the applications create a database with the OS
> username as some default procedure?

it's not that database name is assume to be operating system user name.

psql (or specifically libpq) makes two assumptions:
1. if username is not given - use operating system user name
2. if database name is not given - use username as db name.

So, if you don't have anything set in environment, and you're logged in
a neha, then:

$ psql
will try to connect to db neha with user neha

$ psql -U depesz
will try to connect to db depesz with user depesz

$ psql -d xxx
will try to connect to db xxx with user nega

$ psql -U depesz -d xxx
will try to connect to db xxx with user depesz

> In my specific case the problem is just that the database 'pgadmin' is not
> there, then why should it be a FATAL error. I understand that any process
> aborting errors are treated FATAL. But it looks like a simple error here.

Levels or errors are clearly defined:
1. ERROR means that something was wrong, but you can still continue to
   work in the db connection
2. FATAL means that something wrong happened, and you can't use the
   connection any more as it has closed.

Since failing to open db connection prohibits you from doing any more
work in the db connection (which is not there) - it's clearly FATAL and
not ERROR.

depesz

On Fri, Mar 24, 2017 at 08:49:11AM +1100, Neha Khatri wrote:
> I had the question specific to above two cases, when the database name is
> not provided. There is no surety of having a database name same as the user
> name. Then why assume the database name and try to connect to it. Is it due
> to legacy reason or due to some common database naming practice?

so what would you prefer to do, as psql, in such case? report error
unconditionally? after all - if user didn't provide db name, and you
don't want to assume db name that is the same as user name - then you
don't know what db to connect to.

Best regards,

depesz

On Fri, Mar 24, 2017 at 08:39:30AM +0000, Neha Khatri wrote:
> Yes, so psql should say "Not sure which database to connect to. Please
> specify a valid database name using -d option".
> For backwards compatibility, psql can still attempt to connect to db with
> name same as the username, but the error message could be more like
> providing the information, what can be done next.

Sorry, but I don't agree.
I use databases named like users all the time, and love the fact that
I don't have to provide dbname when it's obvious.

Error message is, in my opinion, clear enough - you're trying to connect
to database that doesn't exist. Why you are trying to - impossible to
tell. Maybe you made a typo. Maybe you forgot to add option. Maybe you
forgot to export env variable.

What is being chosen as user/database name is documented in psql docs.
Additionally - if you want to require that user provides database name,
why don't require also host and port? After all, you need all
four parameters to connect: host, port, username and database name.

If all you ever want is to connect to db named "x", then why not using
psql -d x, or even setting PGDATABASE to x?

depesz

On Mon, Mar 27, 2017 at 09:37:44AM +1100, Neha Khatri wrote:
> > Sorry, but I don't agree.
> > I use databases named like users all the time, and love the fact that
> > I don't have to provide dbname when it's obvious.
> So, is it common practice to have a database name same as the username.
> What are the usecase advantages of the database name and username mapping?
> It would be good to understand.

Why wouldn't I use the name of user?

I can have multiple users on a servers that want to "play" with
database, so using db name the same as user name makes it trivial to use
and figure out who is using which db.

Best regards,

depesz