Re: PostgreSQL 12.4 Parallel Query doesn't work while EXPLAIN is OK

Thanks for the ideas, I tested them:
>
> The essential question here is when the function is executed, so you should use
> EXPLAIN (VERBOSE) to see that.

Append (cost=0.00..5.12 rows=4 width=32)
-> Gather (cost=0.00..1.26 rows=1 width=32)
Output: plpgsql_function(...)
Workers Planned: 8
-> Parallel Seq Scan on public.table t (cost=0.00..1.01 rows=1 width=174)
Output: ...
Filter: (t.id <http://t.id/> = 1)
-> Gather (cost=0.00..1.26 rows=1 width=32)
Output: plpgsql_function(...)
Workers Planned: 8
-> Parallel Seq Scan on public.table t_1 (cost=0.00..1.01 rows=1 width=174)
Output: ...
Filter: (t_1.id <http://t_1.id/> = 2)

> Possible explanations:
>
> - The function is executed after the "Gather" node.

The question is - could we do something to fix it?

> Perhaps you didn't define it as PARALLEL SAFE.

The function is marked as "PARALLEL RESTRICTED” because it’s uses temp tables (and I tested it as PARALLEL SAFE with the same result… parallelisation doesn’t work anyway).

>
> - Perhaps the tables are small.

Yes, but these settings applied when the table is created:

analyze table;
set parallel_setup_cost = 0;
set parallel_tuple_cost = 0;
set force_parallel_mode = on;
alter table table set (parallel_workers = 8);

P.S. Actually, I just need to run in parallel mode one function with a set of different arguments to utilise all available CPUs. That’s strange but I couldn’t google a way to do it.