Re: BUG #14691: Isolation failure in deferrable transaction concurrent with schema change

On Mon, Jun 5, 2017 at 12:11 PM, <cpacejo(at)clearskydata(dot)com> wrote:

> The following bug has been logged on the website:
>
> Bug reference: 14691
> Logged by: Chris Pacejo
> Email address: cpacejo(at)clearskydata(dot)com
> PostgreSQL version: 9.5.7
> Operating system: CentOS 7
> Description:
>
> Hi, issuing a SERIALIZABLE DEFERRABLE READ ONLY read from a table to which
> a
> column is being added results in an isolation failure.
>

> me=> BEGIN; ALTER TABLE foo ADD COLUMN y integer DEFAULT 0;
>

> me=> BEGIN; ALTER TABLE foo ADD COLUMN y integer; UPDATE foo SET y = 0;
>

> x | y
> ---+---
> 1 |
> 2 |
> 3 |
> (3 rows)
>

​This is a documented limitation.

​https://www.postgresql.org/docs/9.5/static/mvcc-caveats.html

"Some DDL commands, currently only TRUNCATE and the table-rewriting forms
of ALTER TABLE, are not MVCC-safe."

The first command is "table-rewriting".

The second ALTER TABLE doesn't rewrite the table but does change its
structure - so you get rows but the newly added column is null. This would
seem to be a reasonable behavior.

David J.