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From "Jacques Nadeau (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (DRILL-4279) Improve query plan when no column is required from SCAN
Date Thu, 28 Jan 2016 05:48:40 GMT

    [ https://issues.apache.org/jira/browse/DRILL-4279?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15120812#comment-15120812
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Jacques Nadeau commented on DRILL-4279:
---------------------------------------

A skipAll query means no columns need to be read. The planner doesn't need any columns from
the reader. It shouldn't request any. All that needs to be returned is the correct number
of records. A reader should be able to determine how to do that most efficiently. If the planner
picks a particular column, then the reader has to return that column. In many cases, this
will be substantially less efficient. 

> Improve query plan when no column is required from SCAN
> -------------------------------------------------------
>
>                 Key: DRILL-4279
>                 URL: https://issues.apache.org/jira/browse/DRILL-4279
>             Project: Apache Drill
>          Issue Type: Bug
>          Components: Query Planning & Optimization
>            Reporter: Jinfeng Ni
>            Assignee: Jinfeng Ni
>
> When query does not specify any specific column to be returned SCAN,  for instance,
> {code}
> Q1:  select count(*) from T1;
> Q2:  select 1 + 100 from T1;
> Q3:  select  1.0 + random() from T1; 
> {code}
> Drill's planner would use a ColumnList with * column, plus a SKIP_ALL mode. However,
the MODE is not serialized / deserialized. This leads to two problems.
> 1).  The EXPLAIN plan is confusing, since there is no way to different from a "SELECT
* " query from this SKIP_ALL mode. 
> For instance, 
> {code}
> explain plan for select count(*) from dfs.`/Users/jni/work/data/yelp/t1`;
> 00-03          Project($f0=[0])
> 00-04            Scan(groupscan=[EasyGroupScan [selectionRoot=file:/Users/jni/work/data/yelp/t1,
numFiles=2, columns=[`*`], files= ... 
> {code} 
> 2) If the query is to be executed distributed / parallel,  the missing serialization
of mode would means some Fragment is fetching all the columns, while some Fragment is skipping
all the columns. That will cause execution error.
> For instance, by changing slice_target to enforce the query to be executed in multiple
fragments, it will hit execution error. 
> {code}
> select count(*) from dfs.`/Users/jni/work/data/yelp/t1`;
> org.apache.drill.common.exceptions.UserRemoteException: DATA_READ ERROR: Error parsing
JSON - You tried to start when you are using a ValueWriter of type NullableBitWriterImpl.
> {code}
> Directory "t1" just contains two yelp JSON files. 
> Ideally, I think when no columns is required from SCAN, the explain plan should show
an empty of column list. The MODE of SKIP_ALL together with star * column seems to be confusing
and error prone. 



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