goldmedal commented on issue #10688:
URL: https://github.com/apache/datafusion/issues/10688#issuecomment-2137711858
> I think it is because the first `-128.2` is read as float 32, then treated
as float 64. I said "treated" because I don't see there is cast expression. It
may be happened when DataFusion parses float literal and resolves its data type.
Agreed. I also found it.
```
> explain select 128.2::float;
+---------------+------------------------------------------------+
| plan_type | plan |
+---------------+------------------------------------------------+
| logical_plan | Projection: Float32(128.2) AS Float64(128.2) |
| | EmptyRelation |
| physical_plan | ProjectionExec: expr=[128.2 as Float64(128.2)] |
| | PlaceholderRowExec |
| | |
+---------------+------------------------------------------------+
2 row(s) fetched.
Elapsed 0.004 seconds.
> explain select 128.2;
+---------------+------------------------------------------------+
| plan_type | plan |
+---------------+------------------------------------------------+
| logical_plan | Projection: Float64(128.2) |
| | EmptyRelation |
| physical_plan | ProjectionExec: expr=[128.2 as Float64(128.2)] |
| | PlaceholderRowExec |
| | |
+---------------+------------------------------------------------+
2 row(s) fetched.
Elapsed 0.004 seconds.
> explain select 128.2::double;
+---------------+------------------------------------------------+
| plan_type | plan |
+---------------+------------------------------------------------+
| logical_plan | Projection: Float64(128.2) |
| | EmptyRelation |
| physical_plan | ProjectionExec: expr=[128.2 as Float64(128.2)] |
| | PlaceholderRowExec |
| | |
+---------------+------------------------------------------------+
2 row(s) fetched.
Elapsed 0.003 seconds.
> explain select -128.2 union all select -128.2::double;
+---------------+----------------------------------------------------+
| plan_type | plan |
+---------------+----------------------------------------------------+
| logical_plan | Union |
| | Projection: Float64(-128.2) AS Float64(-128.2) |
| | EmptyRelation |
| | Projection: Float64(-128.2) AS Float64(-128.2) |
| | EmptyRelation |
| physical_plan | UnionExec |
| | ProjectionExec: expr=[-128.2 as Float64(-128.2)] |
| | PlaceholderRowExec |
| | ProjectionExec: expr=[-128.2 as Float64(-128.2)] |
| | PlaceholderRowExec |
| | |
+---------------+----------------------------------------------------+
2 row(s) fetched.
Elapsed 0.008 seconds.
> explain select -128.2 union all select -128.2::float;
+---------------+----------------------------------------------------------------+
| plan_type | plan
|
+---------------+----------------------------------------------------------------+
| logical_plan | Union
|
| | Projection: Float64(-128.2) AS Float64(-128.2)
|
| | EmptyRelation
|
| | Projection: Float64(-128.1999969482422) AS
Float64(-128.2) |
| | EmptyRelation
|
| physical_plan | UnionExec
|
| | ProjectionExec: expr=[-128.2 as Float64(-128.2)]
|
| | PlaceholderRowExec
|
| | ProjectionExec: expr=[-128.1999969482422 as
Float64(-128.2)] |
| | PlaceholderRowExec
|
| |
|
+---------------+----------------------------------------------------------------+
2 row(s) fetched.
Elapsed 0.008 seconds.
````
If we cast `128.2` to `double`, the value won't be changed. I guess the
reason is that DataFusion performs type coercion between float32 and float64
when combining two select plans.
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