问题是我在一个非常简单的查询中,针对一个非常琐碎的设置,总是得到序列扫描。我做错了什么吗?
- Windows Server 2016 上的Postgres 11
- 所做的配置更改:
constraint_exclusion = partition
- 一个单表分成了200个子表,每个分区有数百万条记录。
- 针对涉及字段的索引(假定一个也被分区)
这是创建语句:
CREATE TABLE A (
K int NOT NULL,
X bigint NOT NULL,
Date timestamp NOT NULL,
fy smallint NOT NULL,
fz decimal(18, 8) NOT NULL,
fw decimal(18, 8) NOT NULL,
fv decimal(18, 8) NULL,
PRIMARY KEY (K, X)
) PARTITION BY LIST (K);
CREATE TABLE A_1 PARTITION OF A FOR VALUES IN (1);
CREATE TABLE A_2 PARTITION OF A FOR VALUES IN (2);
...
CREATE TABLE A_200 PARTITION OF A FOR VALUES IN (200);
CREATE TABLE A_Default PARTITION OF A DEFAULT;
CREATE INDEX IX_A_Date ON A (Date);
需要翻译的内容:
涉及的查询:
SELECT K, MIN(Date), MAX(Date)
FROM A
GROUP BY K
这总是会产生一个序列扫描,需要几分钟的时间,尽管很明显根本不需要表数据,因为日期字段已被索引,并且我只是在请求其B树的第一个和最后一个叶子节点。
最初索引是在 (K, Date)
上,它迅速向我呈现了Postgres不会在任何查询中使用我认为会使用的索引。在 (Date)
上的索引为其他查询提供了解决方案,似乎Postgres声称自动对索引进行分区。然而,这个特定的简单查询总是进行顺序扫描。
欢迎任何想法!
更新
查询计划 (分析、缓冲区)
如下:
Finalize GroupAggregate (cost=4058360.99..4058412.66 rows=200 width=20) (actual time=148448.183..148448.189 rows=5 loops=1)
Group Key: a_16.k
Buffers: shared hit=5970 read=548034 dirtied=4851 written=1446
-> Gather Merge (cost=4058360.99..4058407.66 rows=400 width=20) (actual time=148448.166..148463.953 rows=8 loops=1)
Workers Planned: 2
Workers Launched: 2
Buffers: shared hit=5998 read=1919356 dirtied=4865 written=1454
-> Sort (cost=4057360.97..4057361.47 rows=200 width=20) (actual time=148302.271..148302.285 rows=3 loops=3)
Sort Key: a_16.k
Sort Method: quicksort Memory: 25kB
Worker 0: Sort Method: quicksort Memory: 25kB
Worker 1: Sort Method: quicksort Memory: 25kB
Buffers: shared hit=5998 read=1919356 dirtied=4865 written=1454
-> Partial HashAggregate (cost=4057351.32..4057353.32 rows=200 width=20) (actual time=148302.199..148302.203 rows=3 loops=3)
Group Key: a_16.k
Buffers: shared hit=5984 read=1919356 dirtied=4865 written=1454
-> Parallel Append (cost=0.00..3347409.96 rows=94658849 width=12) (actual time=1.678..116664.051 rows=75662243 loops=3)
Buffers: shared hit=5984 read=1919356 dirtied=4865 written=1454
-> Parallel Seq Scan on a_16 (cost=0.00..1302601.32 rows=42870432 width=12) (actual time=0.320..41625.766 rows=34283419 loops=3)
Buffers: shared hit=14 read=873883 dirtied=14 written=8
-> Parallel Seq Scan on a_19 (cost=0.00..794121.94 rows=26070794 width=12) (actual time=0.603..54017.937 rows=31276617 loops=2)
Buffers: shared read=533414
-> Parallel Seq Scan on a_20 (cost=0.00..447025.50 rows=14900850 width=12) (actual time=0.347..52866.404 rows=35762000 loops=1)
Buffers: shared hit=5964 read=292053 dirtied=4850 written=1446
-> Parallel Seq Scan on a_18 (cost=0.00..198330.23 rows=6450422 width=12) (actual time=4.504..27197.706 rows=15481014 loops=1)
Buffers: shared read=133826
-> Parallel Seq Scan on a_17 (cost=0.00..129272.31 rows=4308631 width=12) (actual time=3.014..18423.307 rows=10340224 loops=1)
Buffers: shared hit=6 read=86180 dirtied=1
...
-> Parallel Seq Scan on a_197 (cost=0.00..14.18 rows=418 width=12) (actual time=0.000..0.000 rows=0 loops=1)
-> Parallel Seq Scan on a_198 (cost=0.00..14.18 rows=418 width=12) (actual time=0.001..0.002 rows=0 loops=1)
-> Parallel Seq Scan on a_199 (cost=0.00..14.18 rows=418 width=12) (actual time=0.001..0.001 rows=0 loops=1)
-> Parallel Seq Scan on a_default (cost=0.00..14.18 rows=418 width=12) (actual time=0.001..0.002 rows=0 loops=1)
Planning Time: 16.893 ms
Execution Time: 148466.519 ms
更新2 仅为避免未来出现“你应该在(K,日期)上建索引”的评论:
如果两个索引都存在,则查询计划完全相同,分析数字相同,甚至缓冲区命中/读取几乎相同。