Re: Parallel Seq Scan

Hi

I have a first query

I looked on EXPLAIN ANALYZE output and the numbers of filtered rows are
differen

postgres=# set max_parallel_degree to 4;
SET
Time: 0.717 ms
postgres=# EXPLAIN ANALYZE select count(*) from xxx where a % 10 = 0;
┌───────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┐
│ QUERY
PLAN │
╞═══════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════╡
│ Aggregate (cost=9282.50..9282.51 rows=1 width=0) (actual
time=142.541..142.541 rows=1 loops=1) │
│ -> Gather (cost=1000.00..9270.00 rows=5000 width=0) (actual
time=0.633..130.926 rows=100000 loops=1) │
│ Number of Workers:
2
│
│ -> Parallel Seq Scan on xxx (cost=0.00..7770.00 rows=5000
width=0) (actual time=0.052..411.303 rows=169631 loops=1) │
│ Filter: ((a % 10) =
0)
│
│ Rows Removed by Filter:
1526399
│
│ Planning time: 0.167
ms
│
│ Execution time: 144.519
ms
│
└───────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┘
(8 rows)

Time: 145.374 ms
postgres=# set max_parallel_degree to 1;
SET
Time: 0.706 ms
postgres=# EXPLAIN ANALYZE select count(*) from xxx where a % 10 = 0;
┌────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┐
│ QUERY
PLAN │
╞════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════╡
│ Aggregate (cost=14462.50..14462.51 rows=1 width=0) (actual
time=163.355..163.355 rows=1 loops=1) │
│ -> Gather (cost=1000.00..14450.00 rows=5000 width=0) (actual
time=0.485..152.827 rows=100000 loops=1) │
│ Number of Workers:
1
│
│ -> Parallel Seq Scan on xxx (cost=0.00..12950.00 rows=5000
width=0) (actual time=0.043..309.740 rows=145364 loops=1) │
│ Filter: ((a % 10) =
0)
│
│ Rows Removed by Filter:
1308394
│
│ Planning time: 0.129
ms
│
│ Execution time: 165.102
ms
│
└────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┘
(8 rows)

Rows removed by filter: 1308394 X 1526399. Is it expected?