Re: Foreign Data Wrapper Handler

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?

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Susan E Hurst
Principal Consultant
Brookhurst Data LLC
Email: susan(dot)hurst(at)brookhurstdata(dot)com
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