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https://issues.apache.org/jira/browse/DRILL-4279?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15106026#comment-15106026
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Jinfeng Ni commented on DRILL-4279:
-----------------------------------
Turns out that the execution error listed above is happening due to fact that
the input file has a schema change : when the plan is executed in distributed
mode, Drill will lose the SKIP_ALL mode, and have to read every column from the
json file, and that will hit the schema change. If the plan is executed in
single mode, then the schema change will not be exposed.
This just confirms that the SKIP_MODE is indeed not serialized, and it could
cause performance overhead, since SCAN operator has to read every columns from
the data source.
> 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
> 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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