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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 16:10:40 GMT

    [ https://issues.apache.org/jira/browse/DRILL-4279?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15121794#comment-15121794
] 

Jacques Nadeau commented on DRILL-4279:
---------------------------------------

If Avro reader fails to handle a skipAll query correctly, we should fix that. We can collaborate
on a branch to fix the readers if you want a hand. Why clutter up the planner rather than
fix the problem at the source?

> 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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