Subject: Re: Postgres_FDW doc doesn't specify TYPE support in Remote Execution Options

From: Rajan Pandey <[email-masked]>
Date: Tue, 9 Apr 2024 20:06:00 +0530

Hi team 😃.

In the
https://www.postgresql.org/docs/current/postgres-fdw.html#POSTGRES-FDW-OPTIONS-REMOTE-EXECUTION
doc, it
mentions that Immutable Functions and Hi can be pushed down using
`extensions` option for foreign server.

But it does not mention TYPE. In the shippable.c
<https://doxygen.postgresql.org/shippable_8c_source.html>/lookup_shippable()
function, I found a comment that indicates that type is also pushed down.
Hece, I have removed TYPE from the comments, assuming that it does not make
sense to push down a data type, and only functions and operations can be
pushed down. Please let me know if my assumption is incorrect. 🙂

I have added my patch file with the mail. Thanks!

On Tue, Apr 9, 2024 at 4:06 PM Laurenz Albe <[email-masked]>
wrote:

> On Tue, 2024-04-09 at 15:49 +0530, Rajan Pandey wrote:
> > I was reading
> https://www.postgresql.org/docs/current/postgres-fdw.html#POSTGRES-FDW-OPTIONS-REMOTE-EXECUTION
> > and found that it mentions that Immutable Functions and Operators can
> > be pushed down using `extensions` option for foreign server.
> >
> > But it does not mention about TYPE. In the shippable.c/lookup_shippable()
> > function, I found that type is also pushed down.
>
> The comment only says that data types may be shippable, but not that
> they are actually shipped.  Can you think of a case where a data type
> would be shipped to a foreign server?  I wrote a foreign data wrapper,
> and I cannot think of such a case.
>
> Perhaps the function comment should be adjusted by removing the parenthesis
> or changing it to "(operator/function/...)".
>
> > Does this require updating the docs? Can I raise a PR to do so? Thank
> you! :)
>
> You would send a patch against the "master" branch to the pgsql-docs list
> for that.
>
> Yours,
> Laurenz Albe
>

-- 
Regards
Rajan Pandey

---

From: Rajan Pandey <[email-masked]>
Date: Wed, 10 Apr 2024 15:54:40 +0530

Hi, please ignore my last email about the patch. :/

I have determined that pushdown of custom data types through fdw in fact
does make sense.

Consider this query:
SELECT * FROM customers WHERE <user_defined_type_col>
<user_defined_operator> <user_defined_function>;

To push down the WHERE clause to the foreign servers, both local and
foreign server must have the user_defined database objects (else the query
won't work on the foreign servers).
For built-in types, operators, functions, Postgres knows that pushdown is
safe as the foreign servers the db objects. But for the user_defined db
objects, postgres won't know. Hence, user_defined_type_col needs to be
pushed down explicitly.

On Tue, Apr 9, 2024 at 8:06 PM Rajan Pandey <[email-masked]>
wrote:

> Hi team 😃.
>
> In the
> https://www.postgresql.org/docs/current/postgres-fdw.html#POSTGRES-FDW-OPTIONS-REMOTE-EXECUTION doc, it

-- 
Regards
Rajan Pandey