x=>log(x/min_parallel_table_scan_size)/log(3)+1worker 表比min_parallel_(index|table)_scan_size值每大3倍,PG增加一个worker进程。Workers进程个数不是基于成本的。循环依赖使得复杂的实现变得困难。相反,规划者使用简单的规则。 可以通过ALTER TABLE … SET (parallel_workers = N)来对某个表指定并行进...
数据库的执行计划,也就是 PostgreSQL 执行 SQL 语句的具体步骤 -- Parallel Seq Scan 表示并行顺序扫描,执行消耗了大量时间 --;由于表中有包含大量数据,而查询只返回一行数据,显然这种方法效率很低。 explain analyze SELECT name FROM test WHERE id = 10000; 1. 2. 3. 4. --如果在 id 列上存在索引,则...
2、计算并行计算的成本,优化器根据CBO原则选择是否开启并行(parallel_setup_cost、parallel_tuple_cost)。 3、强制开启并行(force_parallel_mode)。 4、根据表级parallel_workers参数决定每个查询的并行度取最小值(parallel_workers, max_parallel_workers_per_gather) 5、当表没有设置parallel_workers参数,并且表的大小...
Workers Planned:4Workers Launched:4-> Partial Aggregate (cost=1439213.15..1439213.16rows=1width=32) (actualtime=5147.238..5147.239rows=1loops=5)-> Parallel Seq Scanonlineitem (cost=0.00..1402428.00rows=14714059width=5) (actualtime=0.037..3601.882rows=11767943loops=5)Filter: (l_shipdate<='1998-...
(actual time=657.516..658.959 rows=3 loops=1)Workers Planned: 2Workers Launched: 2-> Partial Aggregate (cost=216018.52..216018.53 rows=1 width=8) (actual time=652.264..652.265 rows=1 loops=3)-> Parallel Seq Scan on t1 (cost=0.00..205601.81 rows=41666...
pg_tables=# show max_parallel_workers_per_gather; max_parallel_workers_per_gather --- 8 (1 row) 再查发现只要1.81秒,over,大功告成! 4.原因分析: Parallel Seq Scan”节点生成用于部分聚合的行。“Partial Aggregate”节点使用 SUM() 减少这些行。最后,每个worker进程的SUM计数器由“Gather”节点收集。
(actualtime=3584.897..3584.981rows=3loops=1)Workers Planned:2Workers Launched:2->PartialAggregate(cost=216018.52..216018.53rows=1width=8)(actualtime=3559.160..3559.160rows=1loops=3)->Parallel Seq Scanont1(cost=0.00..205601.81rows=4166681width=0)(actualtime=0.193..3373.351rows=3333333loops=3)...
-> Parallel Seq Scan on public.test_tbl (cost=0.00..517856.98 rows=4166598 width=0) (actual time=0.029..662.165 rows=3333333 loops=3) Buffers: shared hit=96 read=476095 Worker 0: actual time=0.026..661.807 rows=3323029 loops=1 Buffers: shared hit=34 read=158206 ...
actual time=681.485..683.319 rows=3 loops=1) Workers Planned: 2 Workers Launched: 2 -> Partial Aggregate (cost=216018.52..216018.53 rows=1 width=8) (actual time=674.079..674.080 rows=1 loops=3) -> Parallel Seq Scan on t1 (cost=0.00..205601.81 rows=4166681...
-> Parallel Seq Scan on t1 (cost=0.00..205601.81 rows=4166681 width=0) (actual time=0.092..405.615 rows=3333333 loops=3) Planning Time: 0.126 ms Execution Time: 658.997 ms (8 rows) 4、预热到数据库缓存中 testdb=# select pg_prewarm('t1', 'buffer', 'main'); ...