cursor loop in PostgreSQL - json

Question

This below code is a cursor in PostgreSQL 9.0. I want to fetch my records by joining more than one table and i am getting JSON data from that join.
So I want to loop those records and parse that json using query something like
SELECT "Dump"->'activities-steps'->0->'value' as "steps"
FROM "ActivitySessionDump" where "Id"=42594321345021288
then i have to get data from this query and insert to some other table like
insert to table name (key,value);
So i prepared one readonly cursor to achieve this task
begin work;
DECLARE
sessionids INSENSITIVE no scroll CURSOR FOR
SELECT asn."Id",asn."UserId",asn."ActivityId",ad."Dump"
FROM "ActivitySession" as asn inner join "ActivitySessionDump" as ad
on asn."Id"=ad."ActivitySessionId"
where asn."CreatedAt" between now() - interval '5 hours' and now() and asn."ActivityId"=1
for read only;
---- i want her loop should start and i will parse a json Dump by executing query--------
--------insert record to another table---------------
---end loop-----------
FETCH next FROM sessionids;
CLOSE sessionids;
COMMIT WORK;
Any help really appreciated.Thanks

Answer 1

Since you cannot loop in SQL, you'll have to use PL/pgSQL, for example with a DO statement.
In your case, that could look like this:
DO
$$DECLARE
asn_id ...;
asn_userid ...;
...
c refcursor;
BEGIN
/* assign the SQL cursor to the refcursor variable */
c := 'sessionids';
LOOP
FETCH c INTO asn_id, asn_userid, ...;
IF NOT FOUND THEN EXIT; END IF;
/* process the result row */
END LOOP;
END;$$;
Of course it is a bit awkward to declare a cursor in SQL and use it in PL/pgSQL.
It might be better to put the statement in a FOR loop like this:
FOR asn_id, asn_userid, ... IN
SELECT ...
LOOP
/* process the result row */
END LOOP;
Maybe you could even squeeze the whole thing into a single INSERT statement, that would be most efficient:
INSERT INTO ...
(SELECT ...);

Answer 2

As far as I can tell, the loop or function is unnecessary. It can be replaced with a simple query using string aggregation:
SELECT string_agg("Dump"->'activities-steps'->0->'value', ',') as steps
FROM "ActivitySessionDump" d
WHERE d."ActivitySessionId" IN (SELECT asn."Id"
FROM "ActivitySession" as asn
join "PersonDataSource" as pd on pd."UserId" = asn."UserId"
where asn."CreatedAt" between now() - interval '5 days' and now()
and asn."ActivityId" = 1
and pd."DataSourceId" = 1);
Unrelated, but: you should really avoid those dreaded quoted identifiers

Answer 3

here is the code for my question and i am unable to
EXECUTE 'SELECT rec."Dump"::json#>''{activities-steps,0}''->>''value'' as steps ' INTO jsonrec; line;
SELECT '{"activities-steps":[{"dateTime":"2016-10-17","value":"4023"}]}'::json#>'{activities-steps,0}'->>'value' as steps;
where as i can execute this code in console.
but inside function i cant.
CREATE OR REPLACE FUNCTION ThirdPartyDataParse()
RETURNS text AS $$
DECLARE
sessionid NO SCROLL CURSOR FOR SELECT asn."Id",asn."UserId",asn."ActivityId",pd."DataSourceId",ad."Dump"::TEXT
FROM "Development"."ActivitySession" as asn inner join "Development"."PersonDataSource" as pd on pd."UserId" = asn."UserId" inner join "Development"."ActivitySessionDump" as ad
on asn."Id"=ad."ActivitySessionId" where asn."CreatedAt" between now() - interval '5 days' and now() and asn."ActivityId"=1 and pd."DataSourceId"=1 for read only;
titles TEXT DEFAULT '';
rec record;
jsonrec record;
BEGIN
OPEN sessionid;
loop
FETCH sessionid INTO rec;
--raise notice '%d',rec."UserId";
if not found then
exit ;
end if;
EXECUTE 'SELECT rec."Dump"::json#>''{activities-steps,0}''->>''value'' as steps ' INTO jsonrec;
titles := titles || ',' || jsonrec."steps";
end loop;
return titles;
END;
$$ LANGUAGE plpgsql;