PostgreSQL和openGauss优化器对一个关联查询的SQL优化改写

时间：2024-05-09 07:53:22

PostgreSQL和openGauss数据库优化器在merge join关联查询的SQL优化改写

    • PostgreSQL 查询计划
    • openGauss 查询计划
    • 拓展对比

看腻了文章就来听听视频讲解吧：https://www.bilibili.com/video/BV1oH4y137P7/

数据库类型 数据库版本
PostgreSQL 16.2
openGauss 6.0

创建测试表和数据

drop table IF EXISTS t_test_1;
drop table IF EXISTS t_test_2;
create table t_test_1 (id int, info text);
insert into t_test_1 select generate_series(1,10000000),'testdb';
create table t_test_2 as select * from t_test_1;
create index idx_t_test_1_id on t_test_1(id);
create index idx_t_test_2_id on t_test_2(id);
vacuum analyze t_test_1;
vacuum analyze t_test_2;

查询SQL

-- Merge Join
explain (analyze,buffers) select count(*) from t_test_1 t1 join t_test_2 t2 on (t1.id=t2.id) where t1.id between 5000000 and 5700000;
-- 等价写法
explain (analyze,buffers) select count(*) from t_test_1 t1 join t_test_2 t2 on (t1.id=t2.id) where t1.id between 5000000 and 5700000 and t2.id between 5000000 and 5700000;

PostgreSQL 查询计划

测试 PostgreSQL 数据库版本为：16.2

-- 关闭并行，方便查看执行计划
set max_parallel_workers = 0;
set max_parallel_workers_per_gather = 0;

image.png

从执行计划实际扫描的行数「 rows 」，可以看到 MergeJoin 会从索引开头全部扫描，直到超过匹配范围,即 t1 表根据where条件的between过滤走的索引扫描获取数据，t2 表从索引开头全部扫描到匹配范围。

openGauss 查询计划

测试 openGauss 数据库版本为：6.0

image.png
相比 PostgreSQL 的优化器，openGauss在这种场景的处理上相对智能点

拓展对比

PostgreSQL数据库调整where过滤条件行数后的执行计划：

-- Hash Join
explain (analyze,buffers) select count(*) from t_test_1 t1 join t_test_2 t2 on (t1.id=t2.id) where t1.id between 5000000 and 5500000;
-- 等价改写
explain (analyze,buffers) select count(*) from t_test_1 t1 join t_test_2 t2 on (t1.id=t2.id) where t1.id between 5000000 and 5500000 and t2.id between 5000000 and 5500000;

image.png

-- Nested Loop
explain (analyze,buffers) select count(*) from t_test_1 t1 join t_test_2 t2 on (t1.id=t2.id) where t1.id between 5000000 and 5200000;
-- 等价改写
explain (analyze,buffers) select count(*) from t_test_1 t1 join t_test_2 t2 on (t1.id=t2.id) where t1.id between 5000000 and 5200000 and t2.id between 5000000 and 5200000;

image.png

openGauss数据库调整where过滤条件行数后的执行计划：

-- PG Hash Join
explain (analyze,buffers) select count(*) from t_test_1 t1 join t_test_2 t2 on (t1.id=t2.id) where t1.id between 5000000 and 5500000;
-- PG Nested Loop
explain (analyze,buffers) select count(*) from t_test_1 t1 join t_test_2 t2 on (t1.id=t2.id) where t1.id between 5000000 and 5200000;

image.png

随着where条件过滤数量的变化，PostgreSQL执行计划选择的方式会跟着有所变化，openGauss都是选择merge join的执行计划，按照merge join针对数据有序的场景到也算是正常

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