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https://issues.apache.org/jira/browse/DRILL-4279?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15120610#comment-15120610
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Jinfeng Ni commented on DRILL-4279:
-----------------------------------
The change is both in planning and record reader. The skipAll mode in
ColumnList is not used any longer. What the planner change is in case it's
schemed table, it will add at least one column to the column list (from
rowType). The execution change is a method getDefaultColumnsToRead() in
AbstractRecordReader, which could be override by individual reader.
Are you saying you want to keep the plan to have "Columns = [ *],
SKIP_MODE=YES" ? To me, it's quite confusing, or misleading.
> 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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