[CALCITE-7679] RelToSqlConverter generates GROUP BY literals for dial…

[ehds]

…ects that do not support them when the constant is hidden by nested Projects

Jira Link

CALCITE-7679

Changes Proposed

Before applying AggregateProjectConstantToDummyJoinRule, normalize nested projects using PROJECT_MERGE and PROJECT_REMOVE.

[mihaibudiu]

It's not obvious to me why this solves the problem. Can you explain?

[ehds]

I added a new UT to explain why we need theses two rules.

For this sql:

SELECT id, employee_id
FROM (
  SELECT id, employee_id
  FROM (
    SELECT employee_id, NULL AS id
    FROM employee
  ) AS t1
) AS t2
GROUP BY id, employee_id

It will be transformed to:

LogicalAggregate(group=[{0, 1}])
  LogicalProject(id=[$1], employee_id=[$0])
    LogicalProject(employee_id=[$0], id=[null:NULL])
      JdbcTableScan(table=[[foodmart, employee]])

AggregateProjectConstantToDummyJoinRule (added by CALCITE-4702） only matches the Agg->Project pattern, however, $1 and $0 are not literals, so the rule does not fire.

To fix it, we should remove unnecessary PROJECT operations by these two rules.

Or if you have a better suggestion, I'll follow up.

[mihaibudiu]

This works for this particular plan, but why does it do in general what you need?

[ehds]

Indeed, this current solution only handles particular cases.

I think a more general fix would be to wrap the input in a subquery when determining needNewSubQuery if any GROUP BY expression contains a literal.

LogicalAggregate(group=[{0, 1}])
  LogicalProject(id=[$1], employee_id=[$0])
    LogicalProject(employee_id=[$0], id=[null:NULL])
      JdbcTableScan(table=[[foodmart, employee]])

Translate this plan into the following SQL form:

SELECT employee_id, id 
   FROM (SELECT employee_id, null as id from foodmart.employee) as t
group by t.employee_id, t.id

And this approach is similar to the fixes used for other cases，such as CALCITE-7655, CALCITE-4491

[mihaibudiu]

In general you should use the Jira to propose a design, and the PR to implement it.
This approach may work.

[mihaibudiu]

I will wait for CI to be green before I review

[mihaibudiu]

You can run most of the ci checks locally using ./gradlew build before you submit a commit

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
86.7% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[mihaibudiu]

I don't see anything about casts here. What is this comment about?

[mihaibudiu]

This generally looks fine