So the general rule of thumb favoring EXISTS/JOINs is holding good. But wait! Do we see a better execution time with the NOT IN clause even with a sub-plan? Yes. PostgreSQL has done excellent optimization, thereby preparing a Hash of sub-plan NOT (hashed SubPlan 1). So PostgreSQL has ...
代码里面注释中,针对in, exists, any的subplan优化器实现也有一些介绍,涉及到性能相关: 实际上exists这里有提到,匹配到第一条就结束,所以评估是否使用哈希表时可能需要的容量很小。 /* * For an EXISTS subplan, tell lower-level planner to expect that only the * first tuple will be retrieved. For ALL ...
同样的结果集合,不同的描述方式(SQL where条件),执行效果和执行计划可能会千差万别。 写好where条件的规则技巧有很多,比如in用exists替换、is null不选择等等,每种技巧都是基于特定的应用场景和内部背景。这里列举一个对条件列不要轻易处理的例子。 如果我们在where条件中书写一些表达式,通常Oracle在SQL预处理前就会...
Semi Join支持支持hash, merge, nestloop几种JOIN方法 --semi Join的操作在EXISTS中有一个返回TRUE的操...
CREATE EXTENSION IF NOT EXISTS vector; 4. 创建测试表(本文以polardb_pg_help_docs为例),用于存储知识库内容。 CREATE TABLE polardb_pg_help_docs ( id bigserial PRIMARY KEY, title text, -- 文档标题 description text, -- 描述 doc_chunk text, -- 文档分块 token_size int, -- 文档分块字数 emb...
AGGREGATEDROP AGGREGATE — 移除一个聚集函数大纲DROP AGGREGATE [ IF EXISTS ] name ( aggregate_...
Streaming replication in PostgreSQL works on log shipping. Every transaction in postgres is written to a transaction log called WAL (write-ahead log) to achieve durability. A slave uses these WAL segments to continuously replicate changes from its master. There exists three mandatory processes – wa...
https://www.nvidia.com/en-in/networking/infiniband/qm8700/ 分布式数据库中大家最关心的一致性问题 1、对于统计分析而言,如果需要精准统计的场景,比如清结算、统计报表,通常跑批期间库是只读的,所以相差10ms级(因为NTP时间差异在10ms数量级)的时间差异不是关键问题。
(100, 50, 100, false) cdtq1 WHERE NOT EXISTS ( SELECT 1 FROM cinema_data_task_queue WHERE cinema_data_task_id = cdtq1.cinema_data_task_id AND attempted_at IS NULL ) ON CONFLICT (cinema_data_task_id) WHERE attempted_at IS NULL DO NOTHING; END IF;END$$LANGUAGE plpgsqlSET work_mem...
droptableifexistsdoctor; createtabledoctor(namevarchar,oncallboolean,remarkvarchar); insertintodoctor(name,oncall,remark)values('Alice',true,''); insertintodoctor(name,oncall,remark)values('Bob',true,''); insertintodoctor(name,oncall,remark)values('Jacky',false,''); ...