Re: Couple of issues with prepared FETCH commands

From: Andrew Gierth <andrew(at)tao11(dot)riddles(dot)org(dot)uk>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: "pgsql-hackers\(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Couple of issues with prepared FETCH commands
Date: 2017-01-11 21:06:33
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>>>>> "Robert" == Robert Haas <robertmhaas(at)gmail(dot)com> writes:

>> 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.

Robert> To me, that sounds like a bug.

So what's the appropriate fix? My suggestion would be to suppress the
result type check entirely for utility statements; EXPLAIN and SHOW
always return the same thing anyway, and both FETCH and EXECUTE are
subject to the issue described. This would mean conceding that the
result descriptor of a prepared FETCH or EXECUTE might change (i.e. a
Describe of the statement might not be useful, though a Describe of an
opened portal would be ok). I think this would result in the most
obviously correct behavior from the client point of view.

Andrew (irc:RhodiumToad)

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