gatesn commented on issue #2581:
URL: https://github.com/apache/datafusion/issues/2581#issuecomment-2574525606
Apologies, I should have checked the example value. 10_000 shows what I mean:
```
explain select x = cast(10000 AS int) from '/tmp/foo.parquet';
+---------------+---------------------------------------------------------------------------------------------------+
| plan_type | plan
|
+---------------+---------------------------------------------------------------------------------------------------+
| logical_plan | Projection: CAST(/tmp/foo.parquet.x AS Int32) =
Int32(10000) AS /tmp/foo.parquet.x = Int64(10000) |
| | TableScan: /tmp/foo.parquet projection=[x]
|
| physical_plan | ProjectionExec: expr=[CAST(x@0 AS Int32) = 10000 as
/tmp/foo.parquet.x = Int64(10000)] |
| | ParquetExec: file_groups={1 group: [[tmp/foo.parquet]]},
projection=[x] |
| |
|
+---------------+---------------------------------------------------------------------------------------------------+
2 row(s) fetched.
Elapsed 0.004 seconds.
```
A side note, but perhaps we're missing a rule somewhere to know that x can
never `= 10000` when it started out as a u8? Perhaps my change in #13736 that
preserves min/max stats through cast expressions?
But we can see in the physical plan the DataFusion cast from `x` to `Int32`,
even though x is stored as an Int32 inside Parquet, is read back into an Int32
Arrow array, and down-casted to an Int8 arrow array, all before being returned
to DataFusion to be cast back up to Int32.
Admittedly, this could be solved by providing a "target type" in the
projection mask, short of full generic projection expression push-down. But it
remains interesting that many file formats have the ability to optimize some
subset of projection expressions. Even the Parquet reader could push-down
projection expressions over dictionary values prior to a full dictionary decode.
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