On 10/09/2015 08:30 PM, Victor Blomqvist wrote:
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.
Out of curiosity more then any else, what happens if you ADD a column instead of DROP a column in the experiment?
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 <mailto:adrian.klaver@xxxxxxxxxxx>> 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.klaver@xxxxxxxxxxx <mailto:adrian.klaver@xxxxxxxxxxx>
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