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He Yongqiang commented on HIVE-908: ----------------------------------- I have seen this before, so i guess we may already have one ticket. Will try to find out. > optimize limit > -------------- > > Key: HIVE-908 > URL: https://issues.apache.org/jira/browse/HIVE-908 > Project: Hadoop Hive > Issue Type: Improvement > Components: Query Processor > Reporter: Namit Jain > Fix For: 0.5.0 > > > If there is a limit, all the mappers have to finish and create 'limit' number > of rows - this can be pretty expensive for a large file. > The following optimizations can be performed in this area: > 1. Start fewer mappers if there is a limit - before submitting a job, the > compiler knows that there is a limit - so, it might be useful to increase the > split size, thereby reducing the number of mappers. > 2. A counter is maintained for the total outputs rows - the mappers can look > at those counters and decide to exit instead of emitting 'limit' number of > rows themselves. > 2. may lead to some bugs because of bugs in counters, but 1. should > definitely help -- This message is automatically generated by JIRA. - You can reply to this email to add a comment to the issue online.