On 06/21/2017 04:28 AM, Pierre Ducroquet wrote:
> Hi
>
> On our production database, we had a small hiccup when playing a SQL
> migration.
> We write them to be as smooth as possible, trying not to interfere with
> running services, but this time, we got a batch of failures.
> I diagnosed the situation, and found out that we were hit by a known "feature"
> of plpgsql.
> Here is, basically, how to produce the problem :
>
> 0) setup
>
> You need a table, alterning type-incompatible fields (that's easier to
> reproduce the issue), and a trigger using these fields.
> For instance :
>
> CREATE TABLE demo_function (id serial, number integer, status text, number2
> integer);
> CREATE OR REPLACE FUNCTION demo_function_serialize_trigger()
> RETURNS trigger
> LANGUAGE plpgsql
> AS $function$
> BEGIN
> INSERT INTO demo_function_target
> SELECT json_build_object(
> 'number', NEW.number,
> 'status', NEW.status,
> 'number2', NEW.number2);
> RETURN NEW;
> END;
> $function$;
> CREATE TRIGGER demo_function_trg AFTER INSERT OR UPDATE ON demo_function FOR
> EACH ROW EXECUTE PROCEDURE demo_function_serialize_trigger();
>
>
>
> 1) background session
>
> Imagine your web-worker, with its persistant SQL connection, doing this kind
> of query :
> INSERT INTO demo_function(number, status, number2) VALUES (1, 'todo', 2);
>
> This will cache the plan for the stored procedure in that session.
>
>
> 2) alter...
>
> In another session, let's do this :
>
> CREATE TYPE demo_status AS ENUM ('todo', 'doing', 'done');
> ALTER TABLE demo_function ADD COLUMN status_enum demo_status;
> UPDATE demo_function SET status_enum = status::demo_status;
> ALTER TABLE demo_function DROP COLUMN status;
> ALTER TABLE demo_function RENAME COLUMN status_enum TO status;
>
> (It should of course be a bit more complicated, with triggers and so on to
> maintain the new column, split update to prevent locking too many rows, but
> let's focus on the issue here)
>
>
> 3) back to the background...
>
> INSERT INTO demo_function(number, status, number2) VALUES (2, 'todo', 3);
>
> ==> This will crash with the following error :
> type of parameter 15 (demo_status) does not match that when preparing the plan
> (text)
>
>
> And that's a simple one, we could have something far uglier.
>
>
> I found a workaround using event triggers to rewrite every function when an
> alter occurs on such a table, but this seems… odd to me. I don't think we are
> doing anything very complicated here, so I'm surprised that nothing has been
> done yet to fix that issue.
https://www.postgresql.org/docs/9.6/static/plpgsql-statements.html#PLPGSQL-STATEMENTS-EXECUTING-DYN
> Did I miss something obvious, or should I report that as a bug and start
> digging PostgreSQL code ?
https://www.postgresql.org/docs/9.6/static/plpgsql-implementation.html#PLPGSQL-PLAN-CACHING
>
>
> Thanks
>
--
Adrian Klaver
adrian.klaver@xxxxxxxxxxx
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