PostgreSQL:使用 LATERAL JOIN 和聚合初始化查询计划时没有分区修剪

Question

我想知道为什么分区修剪不适用于横向连接和聚合。 下面是我在PostgreSQL 12.1(Ubuntu 12.1-1.pgdg18.04+1(上测试的示例：

CREATE TABLE demo(key BIGINT) PARTITION BY LIST (key);
CREATE TABLE demo_key_1 PARTITION OF demo FOR VALUES IN (1);
CREATE TABLE demo_key_2 PARTITION OF demo FOR VALUES IN (2);
INSERT INTO demo(key) VALUES (1), (2);
ANALYZE demo;

CREATE TABLE demo2(key BIGINT) PARTITION BY LIST (key);
CREATE TABLE demo2_key_1 PARTITION OF demo2 FOR VALUES IN (1);
CREATE TABLE demo2_key_2 PARTITION OF demo2 FOR VALUES IN (2);
INSERT INTO demo2(key) VALUES (1), (2);
ANALYZE demo2;

现在，如果在横向连接下的 SELECT 中没有聚合，则所有内容 按预期工作 - 仅扫描每个表的单个分区：

EXPLAIN ANALYZE
SELECT * FROM demo
JOIN LATERAL (
SELECT key AS key2
FROM demo2
WHERE demo2.key = demo.key
) d ON TRUE
WHERE demo.key = 1;
QUERY PLAN
-----------------------------------------------------------------------------------------------------------
Nested Loop  (cost=0.00..2.03 rows=1 width=16) (actual time=0.007..0.008
rows=1 loops=1)
->  Seq Scan on demo_key_1  (cost=0.00..1.01 rows=1 width=8) (actual
time=0.004..0.005 rows=1 loops=1)
Filter: (key = 1)
->  Seq Scan on demo2_key_1  (cost=0.00..1.01 rows=1 width=8) (actual
time=0.001..0.001 rows=1 loops=1)
Filter: (key = 1)
Planning Time: 0.191 ms
Execution Time: 0.025 ms
(7 rows)

但是，当我尝试包含聚合的非常相似的查询时 函数，demo2 的分区不会从查询计划中修剪：

EXPLAIN ANALYZE
SELECT * FROM demo
JOIN LATERAL (
SELECT sum(demo2.key) AS sum2
FROM demo2
WHERE demo2.key = demo.key
) d ON TRUE
WHERE demo.key = 1;
QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------
Nested Loop  (cost=2.03..3.07 rows=1 width=40) (actual time=0.018..0.018
rows=1 loops=1)
->  Seq Scan on demo_key_1  (cost=0.00..1.01 rows=1 width=8) (actual
time=0.005..0.005 rows=1 loops=1)
Filter: (key = 1)
->  Aggregate  (cost=2.03..2.04 rows=1 width=32) (actual
time=0.011..0.011 rows=1 loops=1)
->  Append  (cost=0.00..2.03 rows=2 width=8) (actual
time=0.004..0.005 rows=1 loops=1)
->  Seq Scan on demo2_key_1  (cost=0.00..1.01 rows=1
width=8) (actual time=0.002..0.002 rows=1 loops=1)
Filter: (key = demo_key_1.key)
->  Seq Scan on demo2_key_2  (cost=0.00..1.01 rows=1
width=8) (never executed)
Filter: (key = demo_key_1.key)
Planning Time: 0.174 ms
Execution Time: 0.082 ms
(11 rows)

当然，demo2_key_2上的Seq Scan从未被执行过，但为什么没有执行呢？ 从查询计划中修剪？具有数百个 分区受此严重影响。

解决方法是向子查询添加冗余条件：

EXPLAIN ANALYZE
SELECT * FROM demo
JOIN LATERAL (
SELECT sum(demo2.key) AS sum2
FROM demo2
WHERE demo2.key = 1 AND demo2.key = demo.key
) d ON TRUE
WHERE demo.key = 1;
QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------
Nested Loop  (cost=1.01..2.05 rows=1 width=40) (actual time=0.011..0.011
rows=1 loops=1)
->  Seq Scan on demo_key_1  (cost=0.00..1.01 rows=1 width=8) (actual
time=0.004..0.004 rows=1 loops=1)
Filter: (key = 1)
->  Aggregate  (cost=1.01..1.02 rows=1 width=32) (actual
time=0.006..0.006 rows=1 loops=1)
->  Result  (cost=0.00..1.01 rows=1 width=8) (actual
time=0.002..0.002 rows=1 loops=1)
One-Time Filter: (demo_key_1.key = 1)
->  Seq Scan on demo2_key_1  (cost=0.00..1.01 rows=1
width=8) (actual time=0.001..0.002 rows=1 loops=1)
Filter: (key = 1)
Planning Time: 0.079 ms
Execution Time: 0.031 ms
(10 rows)

Answer 1

优化器不够聪明，无法推断它可以跳过分区。但由于 v12 中有运行时分区修剪，因此执行程序会跳过扫描分区。

这应该不会对查询性能产生明显的负面影响。

优化器的智能是有限的。如果要进行分区修剪，最好使用简单的条件。