From: Andrew Gierth <andrew@tao11.riddles.org.uk>
To: pgsql-bugs@lists.postgresql.org,  v.g.baranoff@gmail.com
Subject: Re: BUG #15336: Wrong cursor's bacward fetch results in select with
 ALL(subquery)
In-Reply-To: <153449812167.1304.1741624125628126322@wrigleys.postgresql.org>
	(PG Bug reporting form's message of "Fri, 17 Aug 2018 09:28:41 +0000")
Message-ID: <87va892tf5.fsf@news-spur.riddles.org.uk>
References: <153449812167.1304.1741624125628126322@wrigleys.postgresql.org>
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Date: Fri, 17 Aug 2018 11:27:46 +0100
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>>>>> "PG" == PG Bug reporting form <noreply@postgresql.org> writes:

 PG> fetch backward 9 from "mycursor";
 PG> commit;
 PG> The result set is wrong (9, 8, 7, 6, 5, 4, 3, 2, 1).
 PG> It seems that selection predicate is ignored.

 PG> This bug has been reproduced with PostgreSQL 9.6 and 10.0

I wonder if we have a contender here for the oldest reported bug in PG
history; while I haven't tested anything older than 9.5, the incorrect
logic seems to date back to the introduction of subqueries in
6.something.

Here is a simpler test case:

begin;
declare foo cursor for select * from generate_series(1,3) i where i <> all (values (2));
fetch all from foo;  -- returns the expected 2 rows
fetch backward all from foo;  -- assertion failure, or incorrect result

The problem is that the scan direction is being set to "backward" in the
EState, and as a result the subquery evaluation is run in the backward
direction too, which obviously doesn't do anything sensible. The
assertion failure is from the tuplestore code complaining about doing a
backward fetch on a tuplestore not initialized for backward access.

I'm really not sure yet what the correct fix is, though.

-- 
Andrew (irc:RhodiumToad)