Обсуждение: Couldn't cast to record[]

Suresh Kumar R <suresh.arsenal29@gmail.com> writes:
> When I query that table I need the pg_typeof(column) as record[] instead of
> composite_type[].

Why?  record[] is considered a pseudotype (polymorphic type),
which is why casting to it doesn't do anything.

            regards, tom lane

On Thu, Mar 3, 2022 at 4:01 AM Suresh Kumar R
<suresh.arsenal29@gmail.com> wrote:
>
> Hi, I created an table with composite type array as datatype for one column.
> When I query that table I need the pg_typeof(column) as record[] instead of
> composite_type[].
> I tried creating a separate function and returning record[], but below error
> is thrown,
>
> ERROR:  PL/pgSQL functions cannot return type record[]
>
> Below is sample for my requirement.
>
> pali=# create type address as (number bigint, city varchar);
> CREATE TYPE
> pali=# create table person(name varchar, addresses address[]);
> CREATE TABLE
> pali=# insert into person values ('Michael', array[(82, 'NYC'),(24,
> 'Dunkirk')]::address[]);
> INSERT 0 1
> pali=# select pg_typeof(addresses::record[]) from person ;
>  pg_typeof
> -----------
>  address[]
>
> Here I expect record[] instead of address[].


Why do you think you need this? If you need variant record storage,
you probably want to go to jsonb, then use jsonb_populate_recordset to
convert back to specific type.

merlin

Suresh Kumar R <suresh.arsenal29@gmail.com> writes:
>    I needed this because, I use postgres backend along with golang using
> pgx driver. When i query address i need to specify OID of
> address[](composite type). In my case i couldnt find the oid of that type
> since its dynamic.

The usual advice is to look it up on the fly.  As an example:

regression=# create type address as (f1 text, f2 text);
CREATE TYPE
regression=# select 'address[]'::regtype;
  regtype  
-----------
 address[]
(1 row)

regression=# select 'address[]'::regtype::oid;
  oid   
--------
 127056
(1 row)

If the client-side stack wants a type OID for a result column,
it's unlikely to be satisfied with a pseudotype OID anyway.
That provides next-to-no useful information about how to
interpret values.  Are you sure you can't just leave the type
unspecified (e.g. zero) in whatever API is giving you trouble?

            regards, tom lane