ehds commented on code in PR #5130:
URL: https://github.com/apache/calcite/pull/5130#discussion_r3670671032
##########
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:
I added a new UT to explain why we need theses two rules.
For this sql:
```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:
```SQL
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](https://issues.apache.org/jira/browse/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.
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