Re: PostgreSQL 10 parenthesized single-column updates can produce errors

Attaching patch... :-/

On Tue, Oct 31, 2017 at 4:27 PM, Rob McColl <rob(at)robmccoll(dot)com> wrote:

> Between 9.6.5 and 10, the handling of parenthesized single-column UPDATE
> statements changed. In 9.6.5, they were treated identically to
> unparenthesized single-column UPDATES. In 10, they are treated as
> multiple-column updates. This results in this being valid in Postgres
> 9.6.5, but an error in Postgres 10:
>
> CREATE TABLE test (id INT PRIMARY KEY, data INT);
> INSERT INTO test VALUES (1, 1);
> UPDATE test SET (data) = (2) WHERE id = 1;
>
> In 10 and the current master, this produces the error:
>
> errmsg("source for a multiple-column UPDATE item must be a sub-SELECT or
> ROW() expression")
>
> I believe that this is not an intended change or behavior, but is instead
> an unintentional side effect of 906bfcad7ba7cb3863fe0e2a7810be8e3cd84fbd
> Improve handling of "UPDATE ... SET (column_list) = row_constructor". (
> https://github.com/postgres/postgres/commit/906bfcad7ba7cb3
> 863fe0e2a7810be8e3cd84fbd).
>
> This is a small patch to the grammar that restores the previous behavior
> by adding a rule to the set_clause rule and modifying the final rule of the
> set_clause rule to only match lists of more then one element. I'm not sure
> if there are more elegant or preferred ways to address this.
>
> Compiled and tested on Ubuntu 17.04 Linux 4.10.0-33-generic x86_64.
>
> Regression test added under the update test to cover the parenthesized
> single-column case.
>
> I see no reason this would affect performance.
>
> Thanks,
> -rob
>
> --
> Rob McColl
> @robmccoll
> rob(at)robmccoll(dot)com
> 205.422.0909 <(205)%20422-0909>
>