Joins in Hadoop has always been a problem for its users: the Map/Reduce framework seems to be specifically designed for group-by aggregation tasks rather than across-table op- erations; on the other hand, join operation in distributed database syste
The lecture discusses the concept of Search Joins. A SearchJoin is a join operation which extends a local table with ad-ditional attributes based on the large corpus of structureddata that is published on the Web in various formats.
Hive on Spark EXPLAIN statement : 讲述了 Common Join / Map join / Bucket Map Join / Sorted Merge Bucket Map Join / skew join 在explain 中的 树结构 。In Hive, command EXPLAIN can be used to show the execution plan of a query. The
language manual has lots of go
对于SQL的Join，在学习起来可能是比较乱的。我们知道，SQL的Join语法有很 多inner的，有outer的，有left的，有时候，对于Select出来的结果集是什么样子 有点不是很清楚。Coding Horror上有一篇文章（实在不清楚为什么Coding Horror也被墙）通过 文氏图 Venn diagrams 解释了SQL的Join。SQL JOINS
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代码如下:select a.f_username from ( SELECT /*+parallel(gu,4)*/distinct gu.f_username FROM t_base_succprouser gu where gu.f_expectenddate > (select trunc(sysdate,’Y’)FROM DUAL) and gu.f_lotid=1 and gu.f_playid=4 and gu.f_paymoney>=1500 ) A left join
