[jira] [Comment Edited] (SPARK-19222) Limit Query Performance issue

["Yadong Qi (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/SPARK-19222?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15823389#comment-15823389 ] 

Yadong Qi edited comment on SPARK-19222 at 1/16/17 2:56 AM:
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Hi [~maropu], sample means `TABLESAMPLE(x ROWS)` or `TABLESAMPLE(x PERCENT)`, the physical plan of `TABLESAMPLE(x ROWS)` is same to LIMIT, so I think you mean `TABLESAMPLE(x PERCENT)`. User's query like `create table t1 as select * from dest1 where phoneNum = 'xxx' limit 10000000` and want to get 10000000 records as more as possible, table t1 will be analyzed later. We don't know the number of records about the subquery `select * from dest1 where phoneNum = 'xxx'`, so we can't know the percent.


was (Author: waterman):
Hi [~maropu], sample means `TABLESAMPLE(x ROWS)` or `TABLESAMPLE(x PERCENT)`, the physical of `TABLESAMPLE(x ROWS)` is same to limit, so I think you mean `TABLESAMPLE(x PERCENT)`. User's query like `create table t1 as select * from dest1 where phoneNum = 'xxx' limit 10000000` and want to get 10000000 records as more as possible, table t1 will be analyzed later. We don't know the number of records about the subquery `select * from dest1 where phoneNum = 'xxx'`, so we can't know the percent.

> Limit Query Performance issue
> -----------------------------
>
>                 Key: SPARK-19222
>                 URL: https://issues.apache.org/jira/browse/SPARK-19222
>             Project: Spark
>          Issue Type: Bug
>          Components: SQL
>    Affects Versions: 2.1.0
>         Environment: Linux/Windows
>            Reporter: Sujith
>            Priority: Minor
>
> Performance/memory bottle neck occurs in the below mentioned query
> case 1:
> create table t1 as select * from dest1 limit 10000000;
> case 2:
> create table t1 as select * from dest1 limit 1000;
> pre-condition : partition count >=10000
> In above cases limit is being added in the terminal of the physical plan 
> == Physical Plan  ==
> ExecutedCommand
>    +- CreateHiveTableAsSelectCommand [Database:spark}, TableName: t2, InsertIntoHiveTable]
>          +- GlobalLimit 10000000
>             +- LocalLimit 10000000
>                +- Project [imei#101, age#102, task#103L, num#104, level#105, productdate#106, name#107, point#108]
>                   +- SubqueryAlias hive
>                      +- Relation[imei#101,age#102,task#103L,num#104,level#105,productdate#106,name#107,point#108] csv  |
> Issue Hints: 
> Possible Bottleneck snippet in limit.scala file under spark-sql package.
>   protected override def doExecute(): RDD[InternalRow] = {
>     val locallyLimited = child.execute().mapPartitionsInternal(_.take(limit))
>     val shuffled = new ShuffledRowRDD(
>       ShuffleExchange.prepareShuffleDependency(
>         locallyLimited, child.output, SinglePartition, serializer))
>     shuffled.mapPartitionsInternal(_.take(limit))
>   }
> As mentioned in above case 1  (where limit value is 10000000 or partition count is > 10000) and case 2(limit value is small(around 1000)), As per the above snippet when the ShuffledRowRDD
> is created by grouping all the limit data from different partitions to a single partition in executer,  memory issue occurs since all the partition limit data will be collected and 
> grouped  in a single partition for processing, in both former/later case the data count  can go very high which can create the memory bottleneck.
> Proposed solution for case 2:
> An accumulator value can be to send to all partitions, all executor will be updating the accumulator value based on the  data fetched , 
> eg: Number of partition = 100, number of cores =10
> Ideally tasks will be launched in a group of 10 task/core, once the first group finishes the tasks driver will check whether the accumulator value is been reached the limit value if its reached then no further tasks will be launched to executors and the result after applying limit will be returned.
> Please let me now for any suggestions or solutions for the above mentioned problems
> Thanks,
> Sujith



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