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From "Paul Rogers (JIRA)" <j...@apache.org>
Subject [jira] [Created] (DRILL-5920) Drill incorrectly projects column aliases to scan operator
Date Wed, 01 Nov 2017 17:15:00 GMT
Paul Rogers created DRILL-5920:
----------------------------------

             Summary: Drill incorrectly projects column aliases to scan operator
                 Key: DRILL-5920
                 URL: https://issues.apache.org/jira/browse/DRILL-5920
             Project: Apache Drill
          Issue Type: Bug
    Affects Versions: 1.10.0
            Reporter: Paul Rogers
            Priority: Major


The {{TestNewTextReader.ensureColumnNameDisplayedinError}} unit test runs this query:
{code}
select max(columns[1]) as col1
from cp.`textinput/input1.csv`
where col1 is not null
{code}
The following appears in the {{SubScan}} for the {{TextFormatPlugin}}:

{noformat}
[`col1`, `columns`[1]]
{noformat}

This is clearly wrong. The actual table column is {{columns}} (and, specifically, element
1.) {{col1} is an alias that should never have been pushed down to the data source because
the data source does not know about aliases.

Further, the projection list makes no distinction between the "real" and "alias" columns,
so, to the data source, both look like real table columns.

The current workaround is to create a nullable int column for {{col1}} which is, presumably,
replaced by a later projection operator.

Because this behavior is wrong, we must think though all the possible failure cases and how
to handle them in this incorrect design. What if the alias matches an (expensive) table column?
What if the alias is the same as some base column in the same query?

{code}
SELECT a as b, b as c FROM ...
{code}

Incorrect name handling may work in many cases, but it does lead to problems because the behavior
is not following the accepted SQL standards.



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