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

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.klaver@aklaver.com