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Re: Foreign Data Wrapper Handler

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Thanks for the detailed instructions, Laurenz!

"The foreign server encapsulates the connection string to access a remote PostgreSQL database. Define one per remote database you want to access."

Where do I define "one per remote database"?.....in pg_hba.conf?

---
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Susan E Hurst
Principal Consultant
Brookhurst Data LLC
Email: susan.hurst@xxxxxxxxxxxxxxxxxx
Mobile: 314-486-3261

On 2020-11-09 11:12, Laurenz Albe wrote:
On Sun, 2020-11-08 at 13:09 -0600, Susan Hurst wrote:
The first pass thru https://www.postgresql.org/docs/12/fdwhandler.html
does not tell me what I think I need to know, but I will digest this
more thoroughly. Maybe I need to understand more of the lingo re:
foreign data wrappers. I do understand that all fdw names must be unique within a database so if I want to connect to more than one foreign db, I
need a different name for each connection. I cannot name each fdw
postgres_fdw. I would like to name the fdws something like:
dbname_to_foreigndbname.

For example, here are 2 possible fdws:

create foreign data wrapper stp_to_geo;
create foreign data wrapper stp_to_metrics;

That syntax creates the fdw and I can create user mappings but I cannot import any foreign schemas into my database. The error message says that
there is no handler for the fdw. That's where I'm stuck.

BTW, I did try using postgres_fdw as a handler...

create foreign data wrapper stp_to_geo handler postgres_fdw;

...but then I got these errors:
ERROR:  function postgres_fdw() does not exist
ERROR:  foreign-data wrapper "stp_to_geo" does not exist

Looks like I need to study a bit more.

This is how you would create a new foreign data wrapper object for PostgreSQL:

  CREATE FOREIGN DATA WRAPPER myfdw
     HANDLER public.postgres_fdw_handler
     VALIDATOR public.postgres_fdw_validator;

This assumes that you installed the extension "postgres_fdw" in schema "public".

But you normally don't have to create a new foreign data wrapper: the one named
"postgres_fdw" that is created by the extension is good enough.
The only reason would be to have a foreign data wrapper with
non-default options,
but since there are no options for "postgres_fdw", that is moot.

So don't do that.

The hierarchy of objects is as follows:

- The foreign data wrapper encapsulates the code required to access the foreign data source. You need only one per database; no need to create a new one.

- The foreign server encapsulates the connection string to access a remote PostgreSQL database. Define one per remote database you want to access.

- The user mapping encapsulates the credentials for a user to access a
foreign server.
  Create one per user and foreign server (or a single one for PUBLIC =
everybody).

- The foreign table describes how a remote table is mapped locally.
  Define one per table that interests you.

Yours,
Laurenz Albe





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