xudong963 opened a new issue #1818:
URL: https://github.com/apache/arrow-datafusion/issues/1818
**Describe the bug**
When union some `select constant1, contant2, ...` similar queries, union
can't produce the result.
**To Reproduce**
```sql
❯ SELECT 1 as col1, 'one' as col2
UNION ALL
SELECT 2, 'two'
UNION ALL
SELECT 3, 'three';
Plan("UNION ALL schemas are expected to be the same")
❯ SELECT 1 , 'one'
UNION ALL
SELECT 2, 'two'
UNION ALL
SELECT 3, 'three';
Plan("UNION ALL schemas are expected to be the same")
```
**Expected behavior**
Let' see results from postgres:
```sql
postgres=# SELECT 1 AS column1, 'one' AS column2
UNION ALL
SELECT 2, 'two'
UNION ALL
SELECT 3, 'three';
column1 | column2
---------+---------
1 | one
2 | two
3 | three
(3 rows)
postgres=# SELECT 1 , 'one'
UNION ALL
SELECT 2, 'two'
UNION ALL
SELECT 3, 'three';
?column? | ?column?
----------+----------
1 | one
2 | two
3 | three
(3 rows)
```
**Additional context**
I have done an easy analysis, the followings are my thought to solve the
issue:
1. We should fix the default name when the expr in projection doesn't have a
name. Let's see an example. `select a from table1`, `a` is the column from
`table1`, so it naturally has a name `a`, but for `select 1`, `1` isn't a
column, it's a `Value` expr, so its name will be `Int64(1)` in current
datatfusion's implementation, the related code is
https://github.com/apache/arrow-datafusion/blob/7be0e268a69ffecbf06823c98ca572733dddb29e/datafusion/src/physical_plan/planner.rs#L91.
So the first thing to solve the issue is to rethink how to create default
name. I think we can keep consistent with postgres, such as `?column?`.
2. Currently, DF judges if the logical plans in `union` have the same schema
by arrow schema's field name
https://github.com/apache/arrow-datafusion/blob/7be0e268a69ffecbf06823c98ca572733dddb29e/datafusion-common/src/dfschema.rs#L251.
After finishing 1, we can add a condition, if the col name is `?column?`, we
decide if logical plans have the same schema by arrow schema's filed datatype.
cc @alamb @houqp
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