Running parallel functions with union all

I'm trying to run 3 functions at the same time using parallelism and union all. I've read that union queries are candidates for parallelism but i can't get it to work.

The query looks like this (the function is the same, but it's on a different schema, but the result is the same even on the same schema)
-- The real function is a medium size sql query with CTEs, but i get the same result with this
CREATE OR REPLACE FUNCTION fn(jsonb)
RETURNS table(
i int
)
LANGUAGE sql
PARALLEL SAFE
SET search_path from current
AS $function$
select 1
$function$;

select * from catalog.fn('{}')
union all
select * from customer01.fn('{}')
union all
select * from customer02.fn('{}')


And i'm using this setting to force parallelism (i've tried different numbers)

I get this plan

```
Gather (cost=0.25..28.00 rows=3000 width=4) (actual time=7.320..10.669 rows=3 loops=1)
Output: fn.i
Workers Planned: 2
Workers Launched: 2
Buffers: shared hit=388
-> Parallel Append (cost=0.25..28.00 rows=1500 width=4) (actual time=2.283..2.313 rows=2 loops=2)
Buffers: shared hit=388
Worker 0: actual time=2.420..2.422 rows=1 loops=1
Buffers: shared hit=193
Worker 1: actual time=2.146..2.205 rows=2 loops=1
Buffers: shared hit=195
-> Function Scan on catalog.fn (cost=0.25..10.25 rows=1000 width=4) (actual time=2.145..2.145 rows=1 loops=1)
Output: fn.i
Function Call: catalog.fn('{}'::jsonb)
Buffers: shared hit=193
Worker 1: actual time=2.145..2.145 rows=1 loops=1
Buffers: shared hit=193
-> Function Scan on customer01.fn fn_1 (cost=0.25..10.25 rows=1000 width=4) (actual time=2.419..2.419 rows=1 loops=1)
Output: fn_1.i
Function Call: customer01.fn('{}'::jsonb)
Buffers: shared hit=193
Worker 0: actual time=2.419..2.419 rows=1 loops=1
Buffers: shared hit=193
-> Functio

You cannot get that at the moment.

The number of workers is hard-coded as log2(n) + 1, where n is the partition count.

There were attempts make that configurable for PostgreSQL v14, but the problem where and how to specify that proved too hard.

See this closely related question that explains more details.

StackExchange Database Administrators Q#307903, answer score: 2