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Date: Tue, 27 Sep 2016 18:04:20 +0000 (UTC)
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Subject: [jira] [Commented] (DRILL-4905) Push down the LIMIT to the parquet
 reader scan to limit the numbers of records read
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archived-at: Tue, 27 Sep 2016 18:04:22 -0000


    [ https://issues.apache.org/jira/browse/DRILL-4905?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15526958#comment-15526958 ] 

ASF GitHub Bot commented on DRILL-4905:
---------------------------------------

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

    https://github.com/apache/drill/pull/597#discussion_r80756694
  
    --- Diff: exec/java-exec/src/main/java/org/apache/drill/exec/store/parquet/ParquetGroupScan.java ---
    @@ -926,16 +944,22 @@ public GroupScan applyLimit(long maxRecords) {
           fileNames.add(rowGroupInfo.getPath());
         }
     
    -    if (fileNames.size() == fileSet.size() ) {
    +    // If there is no change in fileSet and maxRecords is >= batchSize, no need to create new groupScan.
    +    if (fileNames.size() == fileSet.size() && (maxRecords >= batchSize) ) {
           // There is no reduction of rowGroups. Return the original groupScan.
           logger.debug("applyLimit() does not apply!");
           return null;
         }
     
    +    // If limit maxRecords is less than batchSize, update batchSize to the limit size.
    +    if (maxRecords < batchSize) {
    +      batchSize = (int) maxRecords;
    --- End diff --
    
    I tried to avoid this casting.  To fix this, have to change code in bunch of places.  I was trying to keep code changes minimal so no new regressions are introduced.   


> Push down the LIMIT to the parquet reader scan to limit the numbers of records read
> -----------------------------------------------------------------------------------
>
>                 Key: DRILL-4905
>                 URL: https://issues.apache.org/jira/browse/DRILL-4905
>             Project: Apache Drill
>          Issue Type: Bug
>          Components: Storage - Parquet
>    Affects Versions: 1.8.0
>            Reporter: Padma Penumarthy
>            Assignee: Padma Penumarthy
>             Fix For: 1.9.0
>
>
> Limit the number of records read from disk by pushing down the limit to parquet reader.
> For queries like
> select * from <table> limit N; 
> where N < size of Parquet row group, we are reading 32K/64k rows or entire row group. This needs to be optimized to read only N rows.
>  


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