Category Archives: research

This post is in response to multiple requests from the PG Conf attendees to provide a “schematic” example of how to build a record set and to convert it to a JSON object (basically to illustrate our “Connecting Galaxies” talk.

Actually, if you click on the link above and download the presentation, you will see an extra slide at the end, which provides the link to the code below… but we know that people do not like “clicking the links” 🙂

So – here is it one more time!

--- Building complex JSON objects from multiple tables --
--- Deriving aggregates json_agg and n_array_agg
--- wrapping an array (of rows) into JSON converted to text for JDBC ----transfer 

— Test aggregation
create table aa (a_id text, a_name text);
insert into aa values (‘1’, ‘Anna’),(‘2′,’Bob’);

—- Aggregation on second level —–
create table bb (b_id text, ab_id text, b_num text, b_ph text);
insert into bb values (‘101’, ‘1’, ‘101-101’, ‘(800)-123’);
insert into bb values (‘1012’, ‘1’, ‘1012102’, ‘(800)-1234’),
( ‘201’, ‘2’, ‘201-201’, ‘(800)-1345′);

— add one more embedded array on the second level —–
create table cc (c_id text, ac_id text, c_st text, c_more text);
insert into cc values
( ’11’, ‘1’ , ‘ stst’, ‘more-more’),
( ’12’, ‘1’ , ‘ swsw’, ‘more-less’);
insert into cc values
( ’21’, ‘2’ , ‘ stst-2’, ‘more-mo2222re’),
( ’22’, ‘2’ , ‘ sws222w’, ‘more-less22’);

—- add third level , dd will be under bb —-

create table dd (d_id text, bd_id text, d_value text);
insert into dd values
( ‘1101’ , ‘101’, ‘dv11’),
( ‘1102’ , ‘101’, ‘dv12’),
( ‘2101’ , ‘201’, ‘dv22’);

—- CTE might be helpful to avoid multiple processing of big top-level tables —-

create or replace
function array_agg_next (agg_sta anyarray, val anyelement, b boolean)
returns anyarray as
$$
declare
out_array alias for $0;
begin
if b then
if agg_sta is null then
out_array := array [val ];
else
out_array := agg_sta || val ;
end if;
else
out_array := agg_sta ;
end if;
return out_array;
END;
$$ LANGUAGE plpgsql;

drop
function array_agg_final (agg_sta anyarray);
create or replace
function array_agg_final (agg_sta anyarray)
returns anyarray as
$$
declare final_array alias for $0;
begin
final_array := agg_sta;
return final_array ;
END;
$$ LANGUAGE plpgsql;

—–======================================================
create or replace
function array_agg_next (agg_sta anyarray, val anyelement, b boolean)
returns anyarray as
$$
begin
if b then
if agg_sta is null
then agg_sta := array [val ];
else agg_sta := agg_sta || val;
end if;
end if;
return agg_sta;
END;
$$ LANGUAGE plpgsql;

drop aggregate n_array_agg (anyelement, boolean);
CREATE AGGREGATE n_array_agg (anyelement, boolean) (
sfunc= array_agg_next,
— FINALFUNC = array_agg_final,
STYPE = anyarray
);

drop type if exists jaa cascade;
create type jdd as (ddi_key text, dd_value text);
create type jcc as (cc_key text, cc_st text, more text);
Create type jbb as (bb_key text, num text, ph text, “DD” jdd[]);
create type jaa as (p_key text, p_name text, “BB” jbb[], “CC” jcc[]);

—- collect rows of a specific type into an array, depends on the output type —–
create or replace
function collect_items (s text)
returns jaa[]
LANGUAGE plpgsql as
$body$
declare
result jaa[];
begin
select array_agg(single_item) into result
from
(select
row (
a_id, a_name,
n_array_agg( row (b_id, b_num, b_ph, d_agg )::jbb , b_id is not null),
n_array_agg( row (c_id, c_st, c_more )::jcc, c_id is not null)
) ::jaa as single_item
from
(
select a_id, a_name, b_id, b_num, b_ph, c_id, c_st, c_more,
n_array_agg (row(d_id, d_value)::jdd, d_id is not null) as d_agg
from
(
select a_id, a_name, b_id, b_num, b_ph, null as c_id, null as c_st, null as c_more, d_id, d_value
from aa join bb on a_id = ab_id
left join dd on b_id = bd_id
UNION ALL
select a_id, a_name, null as b_id, null as b_num, null as b_ph, c_id, c_st, c_more, null as d_id, null as d_value
from aa join cc on a_id = ac_id
) temp_table_3
group by
a_id, a_name, b_id, b_num, b_ph, c_id, c_st, c_more
) temp_table_2
group by a_id, a_name) items
where 0=1
;
return result;
end;
$body$;

—– just another output type —-
create or replace
function collect_subitems (s text)
returns jbb[]
LANGUAGE plpgsql as
$body$
declare
result jbb[];
begin
select array_agg(single_item) into result
from
(
select row(
b_id, b_num, b_ph, n_array_agg (row(d_id, d_value)::jdd, d_id is not null)
)::jbb as single_item
from bb left join dd on b_id = bd_id
group by b_id, b_num, b_ph
) items;
return result;
end;
$body$;

— wrap array into a set of text strings containing the json representation of array elements
— This function does not depend on any type
create or replace
function array_transport (all_items anyarray) returns setof text
RETURNS NULL ON NULL INPUT
LANGUAGE plpgsql as
$body$
declare
item record;
begin
foreach item in array all_items
loop
return next( to_json(item))::text;
end loop;
end;
$body$;

select * from array_transport (collect_items(‘a’));

select * from array_transport (collect_subitems(‘a’));

===============================