openGauss学习笔记-278 openGauss性能调优-实际调优案例07-改写SQL消除子查询（案例2）278.1 现象描述278.2 优化说明
openGauss学习笔记-278 openGauss性能调优-实际调优案例07-改写SQL消除子查询（案例2）
278.1 现象描述
如下SQL语句：
UPDATE normal_date n SET time = (
SELECT time FROM normal_date_part p WHERE p.id = n.id
)
WHERE EXISTS
(SELECT 1
FROM normal_date_part n2
WHERE n2.id = n.id);
计划为：
QUERY PLAN
----------------------------------------------------------------------------------------------------------------------------------------------------------------
Update on normal_date n (cost=224.40..2334150.22 rows=5129 width=16) (actual time=17.336..42944.734 rows=10000 loops=1)
-> Hash Semi Join (cost=224.40..2334150.22 rows=5129 width=16) (actual time=16.997..42852.967 rows=10000 loops=1)
Hash Cond: (n.id = n2.id)
-> Seq Scan on normal_date n (cost=0.00..160.29 rows=5129 width=10) (actual time=0.113..7.271 rows=10000 loops=1)
-> Hash (cost=160.29..160.29 rows=5129 width=10) (actual time=7.381..7.381 rows=10000 loops=1)
Buckets: 32768 Batches: 1 Memory Usage: 430kB
-> Seq Scan on normal_date n2 (cost=0.00..160.29 rows=5129 width=10) (actual time=0.052..3.501 rows=10000 loops=1)
SubPlan 1
-> Partition Iterator (cost=0.00..455.00 rows=1 width=8) (actual time=21006.481..42756.884 rows=10000 loops=10000)
Iterations: 331
-> Partitioned Seq Scan on normal_date_part p (cost=0.00..455.00 rows=1 width=8) (actual time=27228.532..27261.944 rows=10000 loops=3310000)
Filter: (id = n.id)
Rows Removed by Filter: 99990000
Selected Partitions: 1..331
Total runtime: 42947.153 ms
(15 rows)
278.2 优化说明
很明显，执行计划中存在SubPlan，并且SubPlan中的运算相当重，即此SubPlan是一个明确的性能瓶颈点。
根据SQL语意等价改写SQL消除SubPlan如下：
update normal_date n set time = (
select time from normal_date_part p where p.id = n.id
);
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