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Date: Tue, 13 Apr 2021 17:01:56 +0300
From: Yura Sokolov <y.sokolov@postgrespro.ru>
To: pgsql-bugs@lists.postgresql.org
Cc: andres@anarezel.de
Subject: Problem with accessing TOAST data in stored procedures
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Message-ID: <03644c0e6bb82132ac783982b6abffdf@postgrespro.ru>
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 <https://www.postgresql.org/message-id/03644c0e6bb82132ac783982b6abffdf%40postgrespro.ru>
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Good day.

There is reproducible bug posted 10 months ago by Konstantin Knizhnik:
https://www.postgresql.org/message-id/flat/5d335911-fb25-60cd-4aa7-a5bd0954aea0%40postgrespro.ru

I'll copy that message here.

> Stored procedure allows to commit/rollback transaction inside its body.
> Unfortunately it is not always correctly handled.
> This fragment of code reports ERROR: no known snapshots

> CREATE TABLE toasted(id serial primary key, data text);
> INSERT INTO toasted(data) VALUES((SELECT string_agg(random()::text,':') 
> FROM generate_series(1, 1000)));
> INSERT INTO toasted(data) VALUES((SELECT string_agg(random()::text,':') 
> FROM generate_series(1, 1000)));
> DO $$ DECLARE v_r record; BEGIN FOR v_r in SELECT data FROM toasted 
> LOOP INSERT INTO toasted(data) VALUES(v_r.data);COMMIT;END LOOP;END;$$;

> I found out that code responsible for persisting portal correctly 
> extracts TOAST data.
> But pl_pgsql is using prefetch and so takes records form SPI_tuptable, 
> not from stored tuplestore.
> I didn't not find better solution rather than disabling prefetch when 
> loop body contains COMMIT or ROLLBACK statements.
> Unfortunately there is no existed walker for plpgsql statements tree, 
> so I have to add such walker.
> I hope that it will be useful not only for this case.
> But may be there are some other ways to fix this problem...
> Please notice the following bug report which may be also related:
> https://www.postgresql.org/message-id/flat/20190904105618.j5l6fhyesmprmstf%40alap3.anarazel.de#d93b550f2f0c1eae785f666251432929

Original message contains patch as well.

Andres, I've mentioned you've participated in GIST case in bottom link, 
and it looks like related,
because COMMIT is called there as well.

Regards,
Yura Sokolov