Re: BUG #18588: Cannot force/let database use parallel execution in simple case.

Ok there are reproducer test case on the stock postgresql 16.4 config:

--good case with small relation
drop table t1;
create table t1 (a int);
insert into t1 select (random()*100)::int from generate_Series(1,1000000)X;
create index on t1(a);
analyze t1;
set min_parallel_index_scan_size to 0;
set min_parallel_table_scan_size to 0;
set parallel_tuple_cost to 0;
set parallel_setup_cost to 0;
set debug_parallel_query to on;
set enable_bitmapscan to 0;
set enable_seqscan to 0;
explain analyze select * from t1 where a<10;

QUERY PLAN

----------------------------------------------------------------------------------------------------------------------------------------------
 Gather  (cost=0.42..19070.70 rows=91047 width=4) (actual
time=0.527..66.086 rows=95306 loops=1)
   Workers Planned: 2
   Workers Launched: 2
   ->  Parallel Index Only Scan using t1_a_idx on t1
(cost=0.42..19070.70 rows=37936 width=4) (actual time=0.046..19.293
rows=31769 loops=3)
         Index Cond: (a < 10)
         Heap Fetches: 95306
 Planning Time: 0.227 ms
 Execution Time: 69.944 ms


--bad case with big relation
--let's make relation 100x larger (and supposedly better suited for
parallel scans)
drop table t1;
create table t1 (a int);
insert into t1 select (random()*10000)::int from generate_Series(1,100000000)X;
create index on t1(a);
analyze t1;
set min_parallel_index_scan_size to 0;
set min_parallel_table_scan_size to 0;
set parallel_tuple_cost to 0;
set parallel_setup_cost to 0;
set debug_parallel_query to on;
set enable_bitmapscan to 0;
set enable_seqscan to 0;
explain analyze select * from t1 where a<10;
explain analyze select * from t1 where a<10;
                                                              QUERY
PLAN

---------------------------------------------------------------------------------------------------------------------------------------
 Gather  (cost=0.57..357316.03 rows=98718 width=4) (actual
time=59.419..651.500 rows=95072 loops=1)
   Workers Planned: 1
   Workers Launched: 1
   Single Copy: true
   ->  Index Only Scan using t1_a_idx on t1  (cost=0.57..357316.03
rows=98718 width=4) (actual time=0.030..585.517 rows=95072 loops=1)
         Index Cond: (a < 10)
         Heap Fetches: 95072
 Planning Time: 0.144 ms
 JIT:
   Functions: 2
   Options: Inlining false, Optimization false, Expressions true, Deforming true
   Timing: Generation 0.396 ms, Inlining 0.000 ms, Optimization 0.000
ms, Emission 0.000 ms, Total 0.396 ms
 Execution Time: 660.359 ms



-- 
Maxim Boguk
Senior Postgresql DBA

Phone UA: +380 99 143 0000
Phone AU: +61  45 218 5678