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

Note that these errors most of the time only happens very briefly at the
same time as the ALTER is run. When I did some experiments today the server
in total had around 3k req/s with maybe 0.1% of them touching the table
being updated, and the error then happens maybe 1-10% of the times I try
this operation. If I do the operation on a table with more load the error
will happen more frequently.

Also, someone suggested me to try and recreate the functions returning the
table as well inside a transaction, but that did not change anything:
BEGIN;
ALTER TABLE...
CREATE OR UPDATE FUNCTION ...
END;

Thanks for your help so far!
/Victor

On Fri, Oct 9, 2015 at 10:49 PM, Adrian Klaver <adrian(dot)klaver(at)aklaver(dot)com>
wrote:

> On 10/09/2015 07:31 AM, Albe Laurenz wrote:
>
>> Adrian Klaver wrote:
>>
>>> For the reason why this is happening see:
>>>>>
>>>>>
>>>>> http://www.postgresql.org/docs/9.4/interactive/plpgsql-implementation.html#PLPGSQL-PLAN-CACHING
>>>>>
>>>>
>>>> Yes, but the ALTER TABLE causes the plan to be recreated the next time.
>>>>
>>>
>>> But does it? From the link above:
>>>
>>> "Because PL/pgSQL saves prepared statements and sometimes execution
>>> plans in this way, SQL commands that appear directly in a PL/pgSQL
>>> function must refer to the same tables and columns on every execution;
>>> that is, you cannot use a parameter as the name of a table or column in
>>> an SQL command. To get around this restriction, you can construct
>>> dynamic commands using the PL/pgSQL EXECUTE statement — at the price of
>>> performing new parse analysis and constructing a new execution plan on
>>> every execution."
>>>
>>> I see '*' as a parameter. Or to put it another way '*' is not referring
>>> to the same thing on each execution when you change the table definition
>>> under the function. Now if I can only get the brain to wake up I could
>>> find the post where Tom Lane explained this more coherently then I can:)
>>>
>>
>> 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.
>>
>
> I stand corrected. I also tried on Postgres 9.3.7, which is a close as I
> could get to OP's 9.3.5 and it worked. Will have to rethink my assumptions.
>
>
>
>> Yours,
>> Laurenz Albe
>>
>>
>
> --
> Adrian Klaver
> adrian(dot)klaver(at)aklaver(dot)com
>