Re: Parallel Seq Scan

On 11 November 2015 at 17:59, Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com> wrote:
> 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?

Yeah, I noticed the same thing, but more pronounced:

With set max_parallel_degree = 4:

# explain (analyse, buffers, timing, verbose, costs) select count(*)
from js where content->'tags'->0->>'term' like 'design%' or
content->'tags'->0->>'term' like 'web%';

QUERY PLAN
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Aggregate (cost=49575.51..49575.52 rows=1 width=0) (actual
time=744.267..744.267 rows=1 loops=1)
Output: count(*)
Buffers: shared hit=175423
-> Gather (cost=1000.00..49544.27 rows=12496 width=0) (actual
time=0.351..731.662 rows=55151 loops=1)
Output: content
Number of Workers: 4
Buffers: shared hit=175423
-> Parallel Seq Scan on public.js (cost=0.00..47294.67
rows=12496 width=0) (actual time=0.030..5912.118 rows=96062 loops=1)
Output: content
Filter: (((((js.content -> 'tags'::text) -> 0) ->>
'term'::text) ~~ 'design%'::text) OR ((((js.content -> 'tags'::text)
-> 0) ->> 'term'::text) ~~ 'web%'::text))
Rows Removed by Filter: 2085546
Buffers: shared hit=305123
Planning time: 0.123 ms
Execution time: 759.313 ms
(14 rows)

With set max_parallel_degree = 0:

# explain (analyse, buffers, timing, verbose, costs) select count(*)
from js where content->'tags'->0->>'term' like 'design%' or
content->'tags'->0->>'term' like 'web%';

QUERY PLAN
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Aggregate (cost=212857.25..212857.26 rows=1 width=0) (actual
time=1235.082..1235.082 rows=1 loops=1)
Output: count(*)
Buffers: shared hit=175243
-> Seq Scan on public.js (cost=0.00..212826.01 rows=12496
width=0) (actual time=0.019..1228.515 rows=55151 loops=1)
Output: content
Filter: (((((js.content -> 'tags'::text) -> 0) ->>
'term'::text) ~~ 'design%'::text) OR ((((js.content -> 'tags'::text)
-> 0) ->> 'term'::text) ~~ 'web%'::text))
Rows Removed by Filter: 1197822
Buffers: shared hit=175243
Planning time: 0.064 ms
Execution time: 1235.108 ms
(10 rows)

Time: 1235.517 ms

Rows removed: 2085546 vs 1197822
Buffers hit: 305123 vs 175243

Thom