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https://issues.apache.org/jira/browse/DRILL-4279?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15121766#comment-15121766
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Jinfeng Ni commented on DRILL-4279:
-----------------------------------
To return a particular column is still much better than to return all the
columns, which is the current behavior. It's easy for planner to specify no
column at all. However, for now, some reader (such as Avro Reader) would
require at least one of its existing column. Otherwise, execution will fail
with such plan. That's why I choose to let planner pick one particular column.
> 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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