Re: Drop or alter column under load give ERROR #42804 structure of query does not match function result type:

Do you have some advice how to design my functions to work around this
problem?

If I understand your conversation correct the problem is returning the
rowtype users from the function. If so, I can think of two workarounds
(both quite inconvenient and complex):

1. Use RETURNS TABLE(...) together with not selecting * in the functions.
2. Use RETURNS <custom type> also without select * in the functions.

What do other people do in this situation? For our system the lowest load
is in the late night, 04 - 06, which might have sufficiently low load to
avoid the issue, but I would much prefer to run schema changes when there
are people in the office.

/Victor

On Mon, Oct 12, 2015 at 10:15 PM, Adrian Klaver <adrian(dot)klaver(at)aklaver(dot)com>
wrote:

> On 10/12/2015 06:53 AM, Tom Lane wrote:
>
>> Andres Freund <andres(at)anarazel(dot)de> writes:
>>
>>> On 2015-10-09 14:32:44 +0800, Victor Blomqvist wrote:
>>>
>>>> CREATE FUNCTION select_users(id_ integer) RETURNS SETOF users AS
>>>> $$
>>>> BEGIN
>>>> RETURN QUERY SELECT * FROM users WHERE id = id_;
>>>> END;
>>>> $$ LANGUAGE plpgsql;
>>>>
>>>
>> My guess is that the problem here is that table level locking prevents
>>> modification of the "users" type when the table is used, but there's no
>>> locking preventing the columns to be dropped while the function is
>>> used. So what happens is that 1) the function is parsed & planned 2)
>>> DROP COLUMN is executed 3) the contained statement is executed 4) a
>>> mismatch between the contained statement and the function definition is
>>> detected.
>>>
>>
>> The query plan as such does get refreshed, I believe. The problem is that
>> plpgsql has no provision for the definition of a named composite type to
>> change after a function's been parsed. This applies to variables of named
>> composite types for sure, and based on this example I think it must apply
>> to the function result type as well, though I'm too lazy to go check the
>> code right now.
>>
>
> That makes sense. The problem is that I cannot square that with Albe's
> example, which I tested also:
>
> "
> Session 1:
>
> test=> CREATE TABLE users (id integer PRIMARY KEY, name varchar NOT NULL,
> to_be_removed integer NOT NULL);
> CREATE TABLE
> test=> CREATE FUNCTION select_users(id_ integer) RETURNS SETOF users AS
> $$BEGIN RETURN QUERY SELECT * FROM users WHERE id = id_; END;$$
> LANGUAGE plpgsql;
> CREATE FUNCTION
>
> Session 2:
>
> test=> SELECT id, name FROM select_users(18);
> id | name
> ----+------
> (0 rows)
>
> Ok, now the plan is cached.
>
> Now in Session 1:
>
> test=> ALTER TABLE users DROP COLUMN to_be_removed;
> ALTER TABLE
>
> Session2:
>
> test=> SELECT id, name FROM select_users(18);
> id | name
> ----+------
> (0 rows)
>
> No error. This is 9.4.4.
> "
>
>
>> We have had past discussions about fixing this. I believe it would
>> require getting rid of use of plpgsql's "row" infrastructure for named
>> composites, at least in most cases, and going over to the "record"
>> infrastructure instead. In the past the conversations have stalled as
>> soon as somebody complained that that would probably make some operations
>> slower. I don't entirely understand that objection, since (a) some other
>> operations would probably get faster, and (b) performance does not trump
>> correctness. But that's where the discussion stands at the moment.
>>
>> regards, tom lane
>>
>>
>>
>
> --
> Adrian Klaver
> adrian(dot)klaver(at)aklaver(dot)com
>