Problems in "SELECT * FROM t SORT BY col1 LIMIT 100"
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Key: HIVE-404
URL: https://issues.apache.org/jira/browse/HIVE-404
Project: Hadoop Hive
Issue Type: Bug
Components: Query Processor
Affects Versions: 0.3.0, 0.4.0
Reporter: Zheng Shao
Unless the user specify "set mapred.reduce.tasks=1;", he will see unexpected
results with the query of "SELECT * FROM t SORT BY col1 LIMIT 100"
Basically, in the first map-reduce job, each reducer will get sorted data and
only keep the first 100. In the second map-reduce job, we will distribute and
sort the data randomly, before feeding into a single reducer that outputs the
first 100.
In short, the query will output 100 random records in N * 100 top records from
each of the reducer in the first map-reduce job.
This is contradicting to what people expects.
We should propagate the SORT BY columns to the second map-reduce job.
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