Jinfeng Ni created DRILL-4279:
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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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