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.klaver@xxxxxxxxxxx> wrote:
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.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.
