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https://issues.apache.org/jira/browse/TRAFODION-2917?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16360951#comment-16360951
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ASF GitHub Bot commented on TRAFODION-2917:
-------------------------------------------

Github user selvaganesang commented on a diff in the pull request:

    https://github.com/apache/trafodion/pull/1417#discussion_r167600705
  
    --- Diff: core/sql/executor/ExHdfsScan.cpp ---
    @@ -118,15 +119,39 @@ ExHdfsScanTcb::ExHdfsScanTcb(
       , dataModCheckDone_(FALSE)
       , loggingErrorDiags_(NULL)
       , loggingFileName_(NULL)
    +  , hdfsClient_(NULL)
    +  , hdfsScan_(NULL)
    +  , hdfsStats_(NULL)
       , hdfsFileInfoListAsArray_(glob->getDefaultHeap(), 
hdfsScanTdb.getHdfsFileInfoList()->numEntries())
     {
       Space * space = (glob ? glob->getSpace() : 0);
       CollHeap * heap = (glob ? glob->getDefaultHeap() : 0);
    +  useLibhdfsScan_ = hdfsScanTdb.getUseLibhdfsScan();
    +  if (isSequenceFile())
    +     useLibhdfsScan_ = TRUE;
       lobGlob_ = NULL;
    -  const int readBufSize =  (Int32)hdfsScanTdb.hdfsBufSize_;
    -  hdfsScanBuffer_ = new(space) char[ readBufSize + 1 ]; 
    -  hdfsScanBuffer_[readBufSize] = '\0';
    -
    +  hdfsScanBufMaxSize_ = hdfsScanTdb.hdfsBufSize_;
    +  headRoom_ = (Int32)hdfsScanTdb.rangeTailIOSize_;
    +
    +  if (useLibhdfsScan_) {
    +     hdfsScanBuffer_ = new(heap) char[ hdfsScanBufMaxSize_ + 1 ]; 
    +     hdfsScanBuffer_[hdfsScanBufMaxSize_] = '\0';
    +  } else {
    +     hdfsScanBufBacking_[0] = new (heap) BYTE[hdfsScanBufMaxSize_ + 2 * 
(headRoom_)];
    --- End diff --
    
    This behavior remains unchanged. Let me check if there is a regression test 
suite already for it


> Refactor Trafodion implementation of hdfs scan for text formatted hive tables
> -----------------------------------------------------------------------------
>
>                 Key: TRAFODION-2917
>                 URL: https://issues.apache.org/jira/browse/TRAFODION-2917
>             Project: Apache Trafodion
>          Issue Type: New Feature
>          Components: sql-general
>            Reporter: Selvaganesan Govindarajan
>            Priority: Major
>             Fix For: 2.3
>
>
> Find below the general outline of hdfs scan for text formatted hive tables.
> Compiler returns a list of scan ranges and the begin range and number of 
> ranges to be done by each instance of TCB in TDB. This list of scan ranges is 
> also re-computed at run time possibly based on a CQD
> The scan range for a TCB can come from the same or different hdfs files.  TCB 
> creates two threads to read these ranges.Two ranges (for the TCB) are 
> initially assigned to these threads. As and when a range is completed, the 
> next range (assigned for the TCB) is picked up by the thread. Ranges are read 
> in multiples of hdfs scan buffer size at the TCB level. Default hdfs scan 
> buffer size is 64 MB. Rows from hdfs scan buffer is processed and moved into 
> up queue. If the range contains a record split, then the range is extended to 
> read up to range tail IO size to get the full row. The range that had the 
> latter part of the row ignores it because the former range processes it. 
> Record split at the file level is not possible and/or not supported.
>  For compression, the compiler returns the range info such that the hdfs scan 
> buffer can hold the full uncompressed buffer.
>  Cons:
> Reader threads feature too complex to maintain in C++
> Error handling at the layer below the TCB is missing or errors are not 
> propagated to work method causing incorrect results
> Possible multiple copying of data
> Libhdfs calls are not optimized. It was observed that the method Ids are 
> being obtained many times. Need to check if this problem still exists.
> Now that we clearly know what is expected, it could be optimized better
>   - Reduced scan buffer size for smoother data flow
>   - Better thread utilization
>   - Avoid multiple copying of data.
> Unable to comprehend the need for two threads for pre-fetch especially when 
> one range is completed fully before the data from next range is processed.
>  Following are the hdfsCalls used by programs at exp and executor directory.
>                   U hdfsCloseFile
>                  U hdfsConnect
>                  U hdfsDelete
>                  U hdfsExists
>                  U hdfsFlush
>                  U hdfsFreeFileInfo
>                  U hdfsGetPathInfo
>                  U hdfsListDirectory
>                  U hdfsOpenFile
>                  U hdfsPread
>                  U hdfsRename
>                  U hdfsWrite
>                  U hdfsCreateDirectory
>  New implementation
>  Make changes to use direct Java APIs for these calls. However, come up with 
> better mechanism to move the data from Java and JNI, avoid unnecessary 
> copying of data, better thread management via Executor concepts in Java. 
> Hence it won’t be direct mapping of these calls to hdfs Java API. Instead, 
> use the abstraction like what is being done for HBase access.
>  I believe newer implementation will be optimized better and hence improved 
> performance. (but not many folds)



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