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

On 10/12/2015 07:53 PM, Victor Blomqvist wrote:
> 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.

Might want to investigate the record return type:

http://www.postgresql.org/docs/9.4/interactive/plpgsql-overview.html#PLPGSQL-ARGS-RESULTS

40.1.2. Supported Argument and Result Data Types

"It is also possible to declare a PL/pgSQL function as returning record,
which means that the result is a row type whose columns are determined
by specification in the calling query, as discussed in Section 7.2.1.4."

The section that explains difference between declared type record and
returned type record:

http://www.postgresql.org/docs/9.4/interactive/plpgsql-declarations.html#PLPGSQL-DECLARATION-RECORDS

How to use a returned record in query:

http://www.postgresql.org/docs/9.4/interactive/queries-table-expressions.html#QUERIES-TABLEFUNCTIONS

See bottom of section.

Basically all the above leaves it up to the calling query to 'shape' the
output. Not sure if that will work for you.

>
> 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 <mailto: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 <mailto: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 <mailto:adrian(dot)klaver(at)aklaver(dot)com>
>
>

--
Adrian Klaver
adrian(dot)klaver(at)aklaver(dot)com