Re: BUG #18314: PARALLEL UNSAFE function does not prevent parallel index build

On Wed, Feb 7, 2024 at 7:54 PM Tender Wang <tndrwang@gmail.com> wrote:
>
> In pg document, I found this:
>
> Also, a block containing an EXCEPTION clause effectively forms a subtransaction that can be rolled back without affecting the outer transaction.
>
> So it will report error for this case even though we replace PARALLEL UNSAFE with PARALLEL SAFE.
>
> I think if PARALLEL UNSAFE is specified by users,  PG internal should not choose to build index parallel, even if the function is
> too simply that can be transform to Const node.
>
> Attached patch will fix Alexander reported issue. I choose pass the raw index expression to is_parallel_safe().
> The comments of RelationGetIndexExpressions() doesn't say it will return optimized expression. So I add a bool argument to RelationGetIndexExpressions().
> I don't decide to write a new function like RelationGetIndexRawExpression. I think the RelationGetIndexExpressions() is enough after adding a bool argument to indicate
> whether caller needing a raw index expression.
>
> But if users specify PARALLEL SAFE, like I said before, it also reports error. But I think it is another thing. Maybe it is reasonable?
>

/home/jian/Downloads/patches/0001-Fix-parallel-index-build-failed.patch:273:
trailing whitespace.
BEGIN
warning: 1 line adds whitespace errors.

+CREATE TABLE btree_para_bld(i int);
+INSERT INTO btree_para_bld SELECT g FROM generate_series(1, 300000) g;
+CREATE INDEX ON btree_para_bld((i + para_unsafe_f()));
you don't need an insert of 300000 rows to test it.

you can just
CREATE TABLE btree_para_bld(i int);
ALTER TABLE btree_para_bld SET (parallel_workers = 4);
SET max_parallel_maintenance_workers TO 4;
CREATE INDEX ON btree_para_bld(i) WHERE i > para_unsafe_f();
reset max_parallel_maintenance_workers;

demo: https://dbfiddle.uk/HpLKN6zj