[jira] [Created] (DRILL-4279) The plan is either confusing or could lead to execution problem, when no columns is required from SCAN

["Jinfeng Ni (JIRA)" <ji...@apache.org>]

Jinfeng Ni created DRILL-4279:
---------------------------------

             Summary: The plan is either confusing or could lead to execution problem, when no columns 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


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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