我有个简单的问题
select count(*)
from taxi_order.ta_orders o
inner join public.t_bases b on b.id = o.id_base
where o.c_phone2 = '012356789'
and b.id_organization = 1
and o.c_date_end < '2017-12-01'::date
group by date_trunc('month', o.c_date_end);
大多数情况下，此查询在不到100毫秒的时间内运行得很快，但有时对于某些c U phone2、id U organization组合，它的运行速度非常慢，可达4秒。
快件执行计划：
HashAggregate (cost=7005.05..7005.62 rows=163 width=8)
Group Key: date_trunc('month'::text, o.c_date_end)
-> Hash Join (cost=94.30..7004.23 rows=163 width=8)
Hash Cond: (o.id_base = b.id)
-> Index Scan using ix_ta_orders_c_phone2 on ta_orders o (cost=0.57..6899.41 rows=2806 width=12)
Index Cond: ((c_phone2)::text = $3)
Filter: (c_date_end < $4)
-> Hash (cost=93.26..93.26 rows=133 width=4)
-> Bitmap Heap Scan on t_bases b (cost=4.71..93.26 rows=133 width=4)
Recheck Cond: (id_organization = $2)
-> Bitmap Index Scan on ix_t_bases_id_organization (cost=0.00..4.68 rows=133 width=0)
Index Cond: (id_organization = $2)
慢案执行计划：
HashAggregate (cost=6604.97..6604.98 rows=1 width=8)
Group Key: date_trunc('month'::text, o.c_date_end)
-> Nested Loop (cost=2195.33..6604.97 rows=1 width=8)
-> Bitmap Heap Scan on t_bases b (cost=2.29..7.78 rows=3 width=4)
Recheck Cond: (id_organization = $2)
-> Bitmap Index Scan on ix_t_bases_id_organization (cost=0.00..2.29 rows=3 width=0)
Index Cond: (id_organization = $2)
-> Bitmap Heap Scan on ta_orders o (cost=2193.04..2199.06 rows=3 width=12)
Recheck Cond: (((c_phone2)::text = $3) AND (id_base = b.id) AND (c_date_end < $4))
-> BitmapAnd (cost=2193.04..2193.04 rows=3 width=0)
-> Bitmap Index Scan on ix_ta_orders_c_phone2 (cost=0.00..58.84 rows=3423 width=0)
Index Cond: ((c_phone2)::text = $3)
-> Bitmap Index Scan on ix_ta_orders_id_base_date_end (cost=0.00..2133.66 rows=83472 width=0)
Index Cond: ((id_base = b.id) AND (c_date_end < $4))
为什么查询计划有时会选择慢而无效的计划？
编辑
表的架构：
craete table taxi_order.ta_orders (
id bigserial not null,
id_base integer not null,
c_phone2 character varying(30),
c_date_end timestamp with time zone,
...
CONSTRAINT pk_ta_orders PRIMARY KEY (id),
CONSTRAINT fk_ta_orders_t_bases REFERENCES public.t_bases (id)
);
craete table public.t_bases (
id serial not null,
id_organization integer not null,
...
CONSTRAINT pk_t_bases PRIMARY KEY (id)
);
塔乌命令~100M行，塔乌基地~2K行。
编辑2
解释慢案分析：
HashAggregate (cost=6355.29..6355.29 rows=1 width=8) (actual time=4075.847..4075.847 rows=1 loops=1)
Group Key: date_trunc('month'::text, o.c_date_end)
-> Nested Loop (cost=2112.10..6355.28 rows=1 width=8) (actual time=114.871..4075.803 rows=2 loops=1)
-> Bitmap Heap Scan on t_bases b (cost=2.29..7.78 rows=3 width=4) (actual time=0.061..0.375 rows=133 loops=1)
Recheck Cond: (id_organization = $2)
Heap Blocks: exact=45
-> Bitmap Index Scan on ix_t_bases_id_organization (cost=0.00..2.29 rows=3 width=0) (actual time=0.045..0.045 rows=133 loops=1)
Index Cond: (id_organization = $2)
-> Bitmap Heap Scan on ta_orders o (cost=2109.81..2115.83 rows=3 width=12) (actual time=30.638..30.638 rows=0 loops=133)
Recheck Cond: (((c_phone2)::text = $3) AND (id_base = b.id) AND (c_date_end < $4))
Heap Blocks: exact=2
-> BitmapAnd (cost=2109.81..2109.81 rows=3 width=0) (actual time=30.635..30.635 rows=0 loops=133)
-> Bitmap Index Scan on ix_ta_orders_c_phone2 (cost=0.00..58.85 rows=3427 width=0) (actual time=0.032..0.032 rows=6 loops=133)
Index Cond: ((c_phone2)::text = $3)
-> Bitmap Index Scan on ix_ta_orders_id_base_date_end (cost=0.00..2050.42 rows=80216 width=0) (actual time=30.108..30.108 rows=94206 loops=133)
Index Cond: ((id_base = b.id) AND (c_date_end < $4))
解释快速案例分析：
HashAggregate (cost=7005.05..7005.62 rows=163 width=8) (actual time=0.927..0.928 rows=1 loops=1)
Group Key: date_trunc('month'::text, o.c_date_end)
-> Hash Join (cost=94.30..7004.23 rows=163 width=8) (actual time=0.903..0.913 rows=2 loops=1)
Hash Cond: (o.id_base = b.id)
-> Index Scan using ix_ta_orders_c_phone2 on ta_orders o (cost=0.57..6899.41 rows=2806 width=12) (actual time=0.591..0.604 rows=4 loops=1)
Index Cond: ((c_phone2)::text = $3)
Filter: (c_date_end < $4)
Rows Removed by Filter: 2
-> Hash (cost=93.26..93.26 rows=133 width=4) (actual time=0.237..0.237 rows=133 loops=1)
Buckets: 1024 Batches: 1 Memory Usage: 13kB
-> Bitmap Heap Scan on t_bases b (cost=4.71..93.26 rows=133 width=4) (actual time=0.058..0.196 rows=133 loops=1)
Recheck Cond: (id_organization = $2)
Heap Blocks: exact=45
-> Bitmap Index Scan on ix_t_bases_id_organization (cost=0.00..4.68 rows=133 width=0) (actual time=0.044..0.044 rows=133 loops=1)
Index Cond: (id_organization = $2)
我知道我可以为每个查询创建单独的索引来加快速度。但我想知道选择错误计划的原因是什么？我的统计数据怎么了？
最佳答案
你必须给我们一个明确的答案。
两个计划之间的区别在于，第二个计划选择嵌套循环连接，因为它估计只有极少数行将从EXPLAIN (ANALYZE, BUFFERS)中选择。由于您抱怨查询速度太慢，所以该估计可能是错误的，从而导致内部表上的循环过多。
尝试通过运行t_bases来改进表统计信息，可能是在增加ANALYZE之后。default_statistics_target上的多列索引将提高嵌套循环计划的执行时间。
关于postgresql - 为什么Postgres选择错误的执行计划，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/48004218/