Re: Automatically parsing in-line composite types

On 29/10/2019 12:23, Dave Cramer wrote:
>
>
> On Wed, 23 Oct 2019 at 15:50, Mitar <mmitar(at)gmail(dot)com
> <mailto:mmitar(at)gmail(dot)com>> wrote:
>
> Hi!
>
> Bump my previous question. I find it surprising that it seems this
> information is not possible to be reconstructed by the client, when
> the server has to have it internally. Is this a new feature request or
> am I missing something?
>
> > I am trying to understand how could I automatically parse an in-line
> > composite type. By in-line composite type I mean a type corresponding
> > to ROW. For example, in the following query:
> >
> > SELECT _id, body, (SELECT array_agg(ROW(comments._id, comments.body))
> > FROM comments WHERE comments.post_id=posts._id) AS comments FROM
> posts
> >
> > It looks like I can figure out that "comments" is an array of
> records.
> > But then there is no way really to understand how to parse those
> > records? So what are types of fields in the record?
> >
> > I start the parsing process by looking at types returned in
> > RowDescription message and then reading descriptions in pg_type
> table.
> >
> > Is there some other way to get full typing information of the
> result I
> > am assuming is available to PostreSQL internally?
>
>
>
> Reading the RowDescription is the only way I am aware of.
>
>
> Dave Cramer
>
> davec(at)postgresintl(dot)com <mailto:davec(at)postgresintl(dot)com>
> www.postgresintl.com <http://www.postgresintl.com>

Perhaps I misunderstood your question, but that sounds like my average
use-case for the object-relational type system & JSON/JSONB
functions/types: defining nested structured types as temporary relations
in my queries and spew out their hierarchical JSON representation -
often as a single big field (ironically I hate storing JSON in
relational databases unless I'm storing something really opaque like
dashboard layouts).

EG:

SELECT
t.relname AS t_name,
array_to_json(ARRAY_AGG(ats)) AS fields_json
FROM
pg_class AS t INNER JOIN (
SELECT
ia.attrelid AS table_id,
ia.attnum AS column_number,
ia.attname AS column_name
FROM
pg_attribute AS ia
) AS ats
ON
(t.relkind = 'r')
AND
(t.relname IN ('pg_type', 'pg_constraint'))
AND
(ats.table_id = t.oid)
GROUP BY
t.relname

You can use subqueries and array_agg() to deepen your output tree all
the way to a stack overflow, a single <whatever>_to_json() call at the
top will recursively traverse and convert whatever you feed it.

In your case you can just emit your composite type as a JSON object or
array thereof (types and relations are the same thing).

--
Regards

Fabio Ugo Venchiarutti
OSPCFC Network Engineering Dpt.
Ocado Technology

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