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From: Diway <diway@diway.net>
To: pgsql-sql@postgresql.org
Subject: Re: json and aggregate
Date: Mon, 4 Nov 2013 13:05:25 -0800 (PST)
Message-ID: <1383599125836-5776914.post@n5.nabble.com> (raw)
In-Reply-To: <1383597496358-5776905.post@n5.nabble.com>
References: <1383596556949-5776903.post@n5.nabble.com>
	<1383597496358-5776905.post@n5.nabble.com>
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David Johnston wrote
> How technical an answer do you want?
> 
> Short answer is that GROUP BY/aggregates cannot process a
> set-returning-function (SRF) in the select-list.  You have move the SRF
> into the associated FROM clause and let the individual rows feed from
> there into the GROUP BY/aggregates.
> 
> From the documentation:
> 
> json_array_elements(json) [returns] SETOF json
> 
> the "SETOF" is the indicator of a SRF.  The reference to "set-valued
> function" is another term for this concept.
> 
> So, yes, the version you do not like as well is required.  First you break
> apart the json THEN you can aggregate.
> 
> David J.

Ok, thanks! I'm pretty new to postgres "advanced" features an didn't even
know what a SRF was :)

Another question if you don't mind, I've written the following function to
convert many field to a json field and then group them based on another id.
I don't really it because I have to query each line...

---
CREATE OR REPLACE FUNCTION fn_tojson() 
    RETURNS TABLE(id integer, data json)
AS
$$
DECLARE 
    current_id integer;
BEGIN
     FOR current_id IN(
       SELECT nah.id FROM item_header nah
     ) LOOP    
            id := current_id;
            
            data := (SELECT json_agg(x) FROM (SELECT message_id, dt_created,
lines, size, current_part, status, fk_item_cat_id FROM item na WHERE
na.fk_item_header_id = $1) x);
            
            RETURN NEXT;
     END LOOP;
END;
$$
LANGUAGE 'plpgsql' VOLATILE;
---

I could use something like this (not tested!)
--
select fk_item_header_id as id, json_agg(row(message_id, dt_created, lines,
size, current_part, status, fk_item_cat_id)) as data from item group by
fk_item_header_id;
--
but my json object keys would be translated to f1, f2, ... and I really need
to keep the original ones.

Thanks again!



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