Re: Shouldn't current_schema() be at least PARALLEL RESTRICTED?

On Thu, Jan 17, 2019 at 9:46 PM Michael Paquier <michael(at)paquier(dot)xyz> wrote:
> While working on the recent issues with 2PC and temporary objects I
> have added a test case based on current_schema() for the first time in
> history, and the buildfarm complained about it, as mentioned here:
> https://www.postgresql.org/message-id/20190118005949.GD1883@paquier.xyz
>
> The has been causing crake and lapwing to complain about
> current_schema() failing to create a temporary schema, which can
> happen when invoked for the first time of a session:
> https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=crake&dt=2019-01-18%2000%3A34%3A20
> https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=lapwing&dt=2019-01-18%2001%3A20%3A01
>
> Here is the problem:
> SET search_path TO 'pg_temp';
> BEGIN;
> SELECT current_schema() ~ 'pg_temp' AS is_temp_schema;
> - is_temp_schema
> -----------------
> - t
> -(1 row)
> -
> +ERROR: cannot create temporary tables during a parallel operation
> PREPARE TRANSACTION 'twophase_search';
> -ERROR: cannot PREPARE a transaction that has operated on temporary namespace
>
> current_schemas() also has this problem.
>
> For now I have stabilized the test by making sure that non-parallel
> plans get used, which makes the buildfarm happy, still that's just a
> workaround in my opinion. One of the reasons why current_schema()
> can create temporary schemas is that there are string dependencies
> with search_path and the way sessions use it, hence it seems to me
> that it would be better to mark the function at least parallel
> restricted on HEAD and avoid any unstable behavior?

It seems like, as currently implemented, the function is
parallel-unsafe, because any inserts, updates, or deletes are
currently parallel-unsafe, including insertions into catalogs. But I
am a bit confused why a function that is called current_schema() ends
up creating a temps schema.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
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