[HIVE-16972] FetchOperator: filter out inputSplits which length is zero

• Type: Improvement
• Status: Resolved
• Priority: Major
• Resolution: Won't Fix
• Affects Version/s: 2.1.0, 2.1.1
• Component/s: Physical Optimizer
• Target Version/s: 3.0.0
• Assignee: Chaozhong Yang
• Reporter: Chaozhong Yang
• Created: 27/Jun/17 07:19
• Updated: 31/Jul/17 01:09
• Resolved: 31/Jul/17 01:09

Description

• Background
  We can describe the basic work flow of common HQL query as follows:
  1. compile and execute
  2. fetch results
  In many cases, we don't need to worry about the issues fetching results from HDFS(iff there are mapreduce jobs generated in planning step). However, the number of results files on HDFS and data distribution will affect the final status of HQL query, especially for HiveServer2. We have some map-only queries, e.g:

  select * from myTable where date > '20170101' and date <= '20170301' and id = 88;
  

  This query will generate more than 20,000 files(look at screenshot image uploaded) on HDFS and most of those files are empty. Of course, they are very sparse. If we send TFetchResultsRequest from HiveServer2 client with some parameters(timeout:90s, maxRows:1024) , FetchOperator can not fetch 1024 rows in 90 seconds and our HiveServer2 client will mark this TFetchResultsRequest as timed out failure. Why? In fact, It's expensive to fetch results from empty file. In our HDFS cluster( 5000+ DataNodes) , reading data from an empty file will cost almost 100 ms (100ms * 1000 ==> 100s > 90s timeout). Obviously, we can filter out those empty files or splits to speed up the process of FetchResults.

Issue Links

• links to: Review Board