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

Thanks for reporting this issue.

I debug the process of creating index, and in plan_create_index_workers()
func, which use the planner to
decide if use parallel building index.

But the index expression passed to is_parallel_safe() has already
simplified by RelationGetIndexExpressions() func.
In this case, FunExpr node will be simplified to Const node. So the check
in is_parallel_safe() will return true.

The goal of plan_create_index_workers() is to decide whether it's safe to
build index parallelly. I think we just check the raw index
expression no need to simplify the index expression.

PG Bug reporting form <noreply(at)postgresql(dot)org> 于2024年1月29日周一 19:02写道：

> The following bug has been logged on the website:
>
> Bug reference: 18314
> Logged by: Alexander Lakhin
> Email address: exclusion(at)gmail(dot)com
> PostgreSQL version: 16.1
> Operating system: Ubuntu 22.04
> Description:
>
> When planner chooses to perform parallel index build, it doesn't
> check whether functions used in an index expression or predicate are
> parallel-safe. For example:
> CREATE FUNCTION f() RETURNS int IMMUTABLE PARALLEL UNSAFE
> AS $$
> BEGIN
> RETURN 0;
> EXCEPTION WHEN OTHERS THEN
> RETURN 1;
> END$$ LANGUAGE plpgsql;
>
> CREATE TABLE t(i int);
> INSERT INTO t SELECT g FROM generate_series(1, 300000) g;
> CREATE INDEX ON t((i + f()));
>
> results in:
> ERROR: cannot start subtransactions during a parallel operation
> CONTEXT: PL/pgSQL function f() line 2 during statement block entry
> parallel worker
>
> Although with 200000 records, the index created with no error.
>
> Reproduced on REL_12_STABLE .. master.
>
>

--
Tender Wang
OpenPie: https://en.openpie.com/