### What changes were proposed in this pull request?
In SPARK-24994 we implemented unwrapping cast for **integral types**. This extends it to support **numeric types** such as float/double/decimal, so that filters involving these types can be better pushed down to data sources.
Unlike the cases of integral types, conversions between numeric types can result to rounding up or downs. Consider the following case:
```sql
cast(e as double) < 1.9
```
assume type of `e` is short, since 1.9 is not representable in the type, the casting will either truncate or round. Now suppose the literal is truncated, we cannot convert the expression to:
```sql
e < cast(1.9 as short)
```
as in the previous implementation, since if `e` is 1, the original expression evaluates to true, but converted expression will evaluate to false.
To resolve the above, this PR first finds out whether casting from the wider type to the narrower type will result to truncate or round, by comparing a _roundtrip value_ derived from **converting the literal first to the narrower type, and then to the wider type**, versus the original literal value. For instance, in the above, we'll first obtain a roundtrip value via the conversion (double) 1.9 -> (short) 1 -> (double) 1.0, and then compare it against 1.9.
<img width="1153" alt="Screen Shot 2020-09-28 at 3 30 27 PM" src="https://user-images.githubusercontent.com/506679/94492719-bd29e780-019f-11eb-9111-71d6e3d157f7.png">
Now in the case of truncate, we'd convert the original expression to:
```sql
e <= cast(1.9 as short)
```
instead, so that the conversion also is valid when `e` is 1.
For more details, please check [this blog post](https://prestosql.io/blog/2019/05/21/optimizing-the-casts-away.html) by Presto which offers a very good explanation on how it works.
### Why are the changes needed?
For queries such as:
```sql
SELECT * FROM tbl WHERE short_col < 100.5
```
The predicate `short_col < 100.5` can't be pushed down to data sources because it involves casts. This eliminates the cast so these queries can run more efficiently.
### Does this PR introduce _any_ user-facing change?
No
### How was this patch tested?
Unit tests
Closes#29792 from sunchao/SPARK-32858.
Lead-authored-by: Chao Sun <sunchao@apple.com>
Co-authored-by: Chao Sun <sunchao@apache.org>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>
feee8da14b
