Re: [HACKERS] Couple of issues with prepared FETCH commands

On Wed, Jan 11, 2017 at 11:28 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Robert Haas <robertmhaas@gmail.com> writes:
>> On Tue, Jan 10, 2017 at 5:38 PM, Andrew Gierth
>> <andrew@tao11.riddles.org.uk> wrote:
>>> But the problem that actually came up is this: if you do the PQprepare
>>> before the named cursor has actually been opened, then everything works
>>> _up until_ the first event, such as a change to search_path, that forces
>>> a revalidation; and at that point it fails with the "must not change
>>> result type" error _even if_ the cursor always has exactly the same
>>> result type.  This happens because the initial prepare actually stored
>>> NULL for plansource->resultDesc, since the cursor name wasn't found,
>>> while on the revalidate, when the cursor obviously does exist, it gets
>>> the actual result type.
>>>
>>> It seems a bit of a "gotcha" to have it fail in this case when the
>>> result type isn't actually being checked in other cases.
>
>> To me, that sounds like a bug.
>
> Yeah --- specifically, I wonder why we allow the reference to an
> unrecognized cursor name to succeed.  Or were you defining the bug
> differently?

I'm not sure whether that's a bug or not.  What I was defining as a
bug is calling a change from "we don't know what the result type will
be" to "we know that the result type will be X" as a change in the
result type.  That's really totally inaccurate.

I've never really understood errors about changing the result type.
As a user, I assumed those were unavoidable implementation artifacts,
on the theory that they were annoying and therefore the developers
would have eliminated such messages had it been practical.  As a
developer, I've never gotten around to understanding whether that
theory was correct.

-- 
Robert Haas
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