phillipleblanc opened a new issue, #15813:
URL: https://github.com/apache/datafusion/issues/15813
### Describe the bug
The DataFusion SQL Unparser does not correctly roundtrip SQL queries that
use `UNION` rather than `UNION ALL`. For example the following query:
```sql
SELECT col1 FROM footable
UNION
SELECT col1 FROM bartable
```
Should result in a query that filters out duplicate rows from the final
result. DataFusion handles this by adding a `LogicalPlan::Distinct` node as the
parent of the `LogicalPlan::Union` node.
```rust
Distinct:
Union:
TableScan
TableScan
```
However, this is currently unparsed to the following SQL:
```sql
SELECT col1 FROM footable
UNION ALL
SELECT col1 FROM bartable
```
That will cause incorrect results when executed, because the duplicate rows
will not be filtered out.
### To Reproduce
Parse the following query into a DataFusion LogicalPlan and then immediately
unparse it and note that it unparses to a `UNION ALL` instead of a `UNION`:
```sql
SELECT j1_string AS col1, j1_id AS id FROM j1
UNION
SELECT j2_string AS col1, j2_id AS id FROM j2
UNION
SELECT j3_string AS col1, j3_id AS id FROM j3
```
### Expected behavior
The `UNION` is correctly preserved in the unparsed SQL from a LogicalPlan
that adds a Distinct node directly above a Union node.
### Additional context
I've already fixed this and will submit a PR shortly.
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