Hi everyone, I am going to do some interesting things for join in hive. Before I read the source code, could anyone tell me what kinds of join have been implemented in the newest version of hive?
Right now, what I have known are: 1. symmetric join has been implemented, which is the default join. 2. asymmetric join, a.k.s. map-side join (for joining a huge table and a small table and only use the map phase), has been implemented.. But no optimization was added. If so, what I think is when we meet two huge tables, we can use semi-join to first get rid of the non-referenced tuples in one tables making it smaller, and then do the map-side join. 3. 3-way join (only use one map-reduce job to join 3 tables) was implemented, but only applied for joining on the same join key (A.k=B.k && B.k=C.k). If we want to join 3 tables on different join keys (A.k1=B..k1 & B.k2=C.k2), we still need 2 map-reduce jobs. 4. when joining two tables, hive could tell whether the join key is a partitioned column, and make good use of this partition feature. 5. no sort-merge join was implemented in hive right now, thus we cannot do the in-equi join. There may be many mistakes in my understanding. Please point it out or give me further information about join in hive. Thanks so much. Luo, Gang --------- Department of Computer Science Duke University (919)316-0993 [email protected] ___________________________________________________________ 好玩贺卡等你发，邮箱贺卡全新上线！ http://card.mail.cn.yahoo.com/
