ehds commented on code in PR #5130:
URL: https://github.com/apache/calcite/pull/5130#discussion_r3671282579
##########
core/src/main/java/org/apache/calcite/rel/rel2sql/SqlImplementor.java:
##########
@@ -194,6 +194,8 @@ protected SqlImplementor(SqlDialect dialect) {
public final Result visitRoot(RelNode r) {
List<RelOptRule> rules = new ArrayList<>();
if (!this.dialect.supportsGroupByLiteral()) {
+ rules.add(CoreRules.PROJECT_MERGE);
Review Comment:
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:
```SQL
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](https://issues.apache.org/jira/browse/CALCITE-7655),
[CALCITE-4491](https://issues.apache.org/jira/browse/CALCITE-4491)
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