Foreign Data Wrapper Handler

On 11/7/20 12:44 PM, Susan Hurst wrote:

Can anyone recommend a good online resource for learning how to set up a
foreign data wrapper using a custom fdw name?  It seems the trick is to
use a handler to make it work but so far the search results have been
elusive for creating a fdw with a successful outcome.

Have you gone through this?:

https://www.postgresql.org/docs/12/fdwhandler.html

Also can we get a definition of 'custom fdw name'?

I'm using 'PostgreSQL 11.8 on amd64-portbld-freebsd12.1, compiled by
FreeBSD clang version 8.0.1 (tags/RELEASE_801/final 366581) (based on
LLVM 8.0.1), 64-bit'

Thanks for your help!

Sue

--
Adrian Klaver
adrian.klaver@aklaver.com

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.

Sue

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

On 11/8/20 11:09 AM, 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;

Aren't you looking for?:

https://www.postgresql.org/docs/12/sql-createserver.html

"
A foreign server typically encapsulates connection information that a
foreign-data wrapper uses to access an external data resource.
Additional user-specific connection information may be specified by
means of user mappings.

The server name must be unique within the database.
"

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.

Sue

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

On 2020-11-07 14:51, Adrian Klaver wrote:

On 11/7/20 12:44 PM, Susan Hurst wrote:

Can anyone recommend a good online resource for learning how to set
up a foreign data wrapper using a custom fdw name?  It seems the
trick is to use a handler to make it work but so far the search
results have been elusive for creating a fdw with a successful outcome.

Have you gone through this?:

https://www.postgresql.org/docs/12/fdwhandler.html

Also can we get a definition of 'custom fdw name'?

I'm using 'PostgreSQL 11.8 on amd64-portbld-freebsd12.1, compiled by
FreeBSD clang version 8.0.1 (tags/RELEASE_801/final 366581) (based on
LLVM 8.0.1), 64-bit'

Thanks for your help!

Sue

--
Adrian Klaver
adrian.klaver@aklaver.com

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
--
Cybertec | https://www.cybertec-postgresql.com

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@brookhurstdata.com
Mobile: 314-486-3261

On Mon, 2020-11-09 at 11:21 -0600, Susan Hurst wrote:

"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?

No, in SQL:

CREATE FOREIGN SERVER somename FOREIFN DATA WRAPPER postgres_fdw OPTIONS (...);

Yours,
Laurenz Albe
--
Cybertec | https://www.cybertec-postgresql.com

On 11/9/20 9:21 AM, Susan Hurst wrote:

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?

https://www.postgresql.org/docs/12/sql-createserver.html

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

--
Adrian Klaver
adrian.klaver@aklaver.com