ReDistribution plan细节

In a Greenplum cluster with 4 segments, when you perform a join between two tables (sales and customer) that are distributed differently, the query plan will involve redistributing data to ensure that related rows are on the same segment. Here's a detailed breakdown of how the redistribution query plan might look:

Tables and Distribution Keys

  • sales table : Distributed by sale_id.

  • customer table : Distributed by cust_id.

Query

sql 复制代码
SELECT s.sale_id, s.amount, c.cust_name
FROM sales s
JOIN customer c ON s.cust_id = c.cust_id;

Query Plan Breakdown

  1. Initial Scan:

    • Each segment scans its local portion of the sales and customer tables.

    • Segment 1 : Scans sales and customer data assigned to it.

    • Segment 2 : Scans sales and customer data assigned to it.

    • Segment 3 : Scans sales and customer data assigned to it.

    • Segment 4 : Scans sales and customer data assigned to it.

  2. Redistribute Motion:

    • Since the sales table is distributed by sale_id and the customer table is distributed by cust_id, the join condition s.cust_id = c.cust_id requires that tuples from sales be redistributed by cust_id.

    • The query plan will include a redistribute motion operator to redistribute the sales table based on cust_id.

  3. Redistribution Execution:

    • The redistribute motion operator will redistribute the sales table across all segments based on the cust_id column.

    • Each segment will receive a portion of the sales table that matches its portion of the customer table.

  4. Local Join:

    • After redistribution, each segment will perform a local join between the redistributed sales data and its local customer data.

    • Segment 1 : Joins redistributed sales data with local customer data.

    • Segment 2 : Joins redistributed sales data with local customer data.

    • Segment 3 : Joins redistributed sales data with local customer data.

    • Segment 4 : Joins redistributed sales data with local customer data.

  5. Gather Motion:

    • The results from each segment are gathered back to the master node.

    • The master node combines the results from all segments to produce the final query result.

Example Query Plan

Here's a simplified example of what the query plan might look like:

复制代码
Gather Motion 4:1  (slice1; segments: 4)
  ->  Hash Join
        Hash Cond: (s.cust_id = c.cust_id)
        ->  Redistribute Motion 4:4  (slice2; segments: 4)
            Hash Key: s.cust_id
            ->  Seq Scan on sales s
        ->  Seq Scan on customer c

Explanation

  1. Gather Motion 4:1:

    • Collects the final results from all 4 segments and combines them on the master node.
  2. Hash Join:

    • Performs a hash join on the cust_id column between the sales and customer tables.
  3. Redistribute Motion 4:4:

    • Redistributes the sales table across all 4 segments based on the cust_id column.
  4. Seq Scan on sales s:

    • Each segment performs a sequential scan on its local portion of the sales table.
  5. Seq Scan on customer c:

    • Each segment performs a sequential scan on its local portion of the customer table.

Conclusion

In this query plan, the redistribution of the sales table based on cust_id ensures that related rows are on the same segment, allowing for efficient local joins. The results from each segment are then gathered back to the master node to produce the final result. This approach leverages Greenplum's MPP architecture to achieve parallel processing and efficient query execution.

相关推荐
自由鬼11 分钟前
开源漏洞扫描器:OpenVAS
运维·服务器·安全·网络安全·开源·漏洞管理
卡皮巴拉爱吃小蛋糕12 分钟前
MySQL的MVCC【学习笔记】
数据库·笔记·mysql
難釋懷13 分钟前
Shell脚本-while循环语法结构
linux·运维·服务器·bash
农民也会写代码14 分钟前
dedecms织梦arclist标签noflag属性过滤多个参数
开发语言·数据库·sql·php·dedecms
m0_7482329220 分钟前
你还在手动画ER图吗?让SQL自动生成ER图,轻松解决作业难题!
数据库·sql·oracle
清流君23 分钟前
【MySQL】数据库 Navicat 可视化工具与 MySQL 命令行基本操作
数据库·人工智能·笔记·mysql·ue5·数字孪生
邂逅岁月23 分钟前
MySQL表的增删改查初阶(下篇)
数据库·sql·mysql
Python_金钱豹24 分钟前
Text2SQL零代码实战!RAGFlow 实现自然语言转 SQL 的终极指南
前端·数据库·sql·安全·ui·langchain·机器人
静听夜半雨26 分钟前
CANoe入门——3、新建LIN工程及LIN DataBase(LDF文件)的创建
网络·数据库·c++·编辑器
B64A-消闲30 分钟前
shell命令一
linux·运维