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

[mobigroup <pechnikov@xxxxxxxxxxxx>]

Hi,

I have strange behaviour when EXPLAIN returns parallel execution plan but the query execution is not parallel. The query looks as

        SELECT

            plpgsql_function(...parameters…)

        FROM table as t

        WHERE id=1

        UNION ALL

        SELECT

            plpgsql_function(...parameters…)

        FROM table as t

        WHERE id=2

        UNION ALL

...

EXPLAIN outputs looks ok:

Append  (cost=0.00..10.25 rows=8 width=32)

  ->  Gather  (cost=0.00..1.27 rows=1 width=32)

        Workers Planned: 8

        ->  Parallel Seq Scan on table t  (cost=0.00..1.27 rows=1 width=32)

              Filter: (id = 1)

  ->  Gather  (cost=0.00..1.27 rows=1 width=32)

        Workers Planned: 8

        ->  Parallel Seq Scan on table t_1  (cost=0.00..1.27 rows=1 width=32)

              Filter: (id = 2)

  ->  Gather  (cost=0.00..1.27 rows=1 width=32)

...

But the query still uses just one core and I see sequential RAISE NOTICE outputs from the function plpgsql_function().

select version();

                                                             version                                                             

---------------------------------------------------------------------------------------------------------------------------------

 PostgreSQL 12.4 (Ubuntu 12.4-1.pgdg18.04+1) on x86_64-pc-linux-gnu, compiled by gcc (Ubuntu 7.5.0-3ubuntu1~18.04) 7.5.0, 64-bit