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From "Cheolsoo Park (JIRA)" <j...@apache.org>
Subject [jira] [Created] (SPARK-9926) Parallelize file listing for partitioned Hive table
Date Thu, 13 Aug 2015 01:40:47 GMT
Cheolsoo Park created SPARK-9926:
------------------------------------

             Summary: Parallelize file listing for partitioned Hive table
                 Key: SPARK-9926
                 URL: https://issues.apache.org/jira/browse/SPARK-9926
             Project: Spark
          Issue Type: Improvement
          Components: SQL
    Affects Versions: 1.4.1, 1.5.0
            Reporter: Cheolsoo Park


In Spark SQL, short queries like {{select * from table limit 10}} run very slowly against
partitioned Hive tables because of file listing. In particular, if a large number of partitions
are scanned on storage like S3, the queries run extremely slowly. Here are some example benchmarks
in my environment-

* Parquet-backed Hive table
* Partitioned by dateint and hour
* Stored on S3

||\# of partitions||\# of files||runtime||query||
|1|972|30 secs|select * from nccp_log where dateint=20150601 and hour=0 limit 10;|
|24|13646|6 mins|select * from nccp_log where dateint=20150601 limit 10;|
|240|136222|1 hour|select * from nccp_log where dateint>=20150601 and dateint<=20150610
limit 10;|

The problem is that {{TableReader}} constructs a separate HadoopRDD per Hive partition path
and group them into a UnionRDD. Then, all the input files are listed sequentially. In other
tools such as Hive and Pig, this can be solved by setting [mapreduce.input.fileinputformat.list-status.num-threads|https://hadoop.apache.org/docs/stable/hadoop-mapreduce-client/hadoop-mapreduce-client-core/mapred-default.xml]
high. But in Spark, since each HadoopRDD lists only one partition path, setting this property
doesn't help.



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