Re: postgres_fdw could deparse ArrayCoerceExpr

Hi.

Alexander Korotkov писал(а) 2025-06-04 14:29:
> On Wed, Jan 29, 2025 at 11:59 AM Maxim Orlov <orlovmg(at)gmail(dot)com> wrote:
>>
>> One important note here. This patch will change cast behaviour in case
>> of local and foreign types are mismatched.
>> The problem is if we cannot convert types locally, this does not mean
>> that it is also true for a foreign wrapped data.
>> In any case, it's up to the committer to decide whether this change is
>> needed or not.
>
> I have two question regarding this aspect.
> 1) Is it the same with regular type conversion?

Yes, it's the same.

CREATE TYPE enum_of_int_like AS enum('1', '2', '3', '4');
CREATE TABLE conversions(id int, d enum_of_int_like);
CREATE FOREIGN TABLE ft_conversions (id int, d char(1))
SERVER loopback options (table_name 'conversions');
SET plan_cache_mode = force_generic_plan;
PREPARE s(varchar) AS SELECT count(*) FROM ft_conversions where d=$1;
EXPLAIN (VERBOSE, COSTS OFF)
EXECUTE s('1');
QUERY PLAN
-------------------------------------------------------------------------------------------
Foreign Scan
Output: (count(*))
Relations: Aggregate on (public.ft_conversions)
Remote SQL: SELECT count(*) FROM public.conversions WHERE ((d =
$1::character varying))
(4 rows)

EXECUTE s('1');
ERROR: operator does not exist: public.enum_of_int_like = character
varying
HINT: No operator matches the given name and argument types. You might
need to add explicit type casts.

> 2) Can we fallback to remote type conversion in local type conversion
> fails?

It's the opposite - we've already planned (and deparsed) statement,
using remote type conversion.
When plan execution fails, there's nothing we can do.
We'll get

PREPARE s(varchar[]) AS SELECT count(*) FROM ft_conversions where
d=ANY($1);
EXPLAIN (VERBOSE, COSTS OFF)
EXECUTE s(ARRAY['1','2']);
QUERY PLAN
---------------------------------------------------------------------------------------------------
Foreign Scan
Output: (count(*))
Relations: Aggregate on (public.ft_conversions)
Remote SQL: SELECT count(*) FROM public.conversions WHERE ((d = ANY
($1::character varying[])))
(4 rows)

EXECUTE s(ARRAY['1','2']);
ERROR: operator does not exist: public.enum_of_int_like = character
varying
HINT: No operator matches the given name and argument types. You might
need to add explicit type casts.

--
Best regards,
Alexander Pyhalov,
Postgres Professional