Re: Lock a viewe

Gaetano Mendola wrote:
> Hi,
> I was wondering if there is a clean view to lock the usage of a view.
>
> Basically during a schema migration, with applications still running a
> typical
> schema change is:
>
> BEGIN;
> ALTER TABLE x ADD COLUMN (a INTEGER);
> CREATE OR REPLACE VIEW v_x
> AS
> SELECT a,b FROM x;
> COMMIT;
>
> now the issue is that if an application performs a:
>
> SELECT * from v_x;
>
> between the ALTER and the view redefinition then a deadlock happens.
>
> I'm preventing this issue doing a:
>
> ALTER VIEW v_x ALTER COLUMN b DROP DEFAULT;
> (anyway there was no default on the view)
> before the ALTER TABLE, that's basically reorders the locks sequence
> avoiding the dead lock.

So what you want is
LOCK VIEW f;
(which is exactly what your hack does) but we don't allow that: you can
only lock tables. I wonder what's the reason for that restriction.
AFAICT changing it is trivial, since it just requires a very minor
change in RangeVarCallbackForLockTable() to allow views.

> Is there a clean way to achieve it without the "hack"?

I can't think of anything.

--
Álvaro Herrera https://www.2ndQuadrant.com/
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