Re: dblink: add polymorphic functions.

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On 07/06/2015 07:37 AM, Merlin Moncure wrote:
> yup, and at least one case now fails where previously it ran
> through: postgres=# select * from dblink('a', 'b', 'c'); ERROR:
> function dblink(unknown, unknown, unknown) is not unique

Hmm, that is an issue, possibly a fatal one.

When Cory first mentioned this to me over a year ago we discussed some
other, arguably better and more generic solutions. One was to build
support for:

SELECT * FROM srf() AS TYPE OF(foo);

The second idea I think is actually SQL standard if I recall correctly:

SELECT * FROM CAST(srf() AS foo) x;

Currently this works:

8<--------------------
select *
from cast (row(11,'l','{a11,b11,c11}') as foo);
f1 | f2 | f3
- ----+----+---------------
11 | l | {a11,b11,c11}
(1 row)
8<--------------------

But this fails:

8<--------------------
select *
from cast
(dblink('dbname=contrib_regression','select * from foo') as foo);
ERROR: cannot cast type record to foo
8<--------------------

Comments in the source have this to say:

8<--------------------
/*
* coerce_record_to_complex
* Coerce a RECORD to a specific composite type.
*
* Currently we only support this for inputs that are RowExprs or
* whole-row Vars.
*/
8<--------------------

That explains why the first example works while the second does not.
I'm not sure how hard it would be to fix that, but it appears that
that is where we should focus.

Joe

- --
Joe Conway
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