Re: Automatically parsing in-line composite types - Mailing list pgsql-general

On 29/10/2019 12:23, Dave Cramer wrote:
> 
> 
> On Wed, 23 Oct 2019 at 15:50, Mitar <mmitar@gmail.com 
> <mailto:mmitar@gmail.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@postgresintl.com <mailto:davec@postgresintl.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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