wangyum opened a new pull request, #37421:
URL: https://github.com/apache/spark/pull/37421
### What changes were proposed in this pull request?
This PR adds support estimate column statistics if it is foldable
expression. For example: estimate the `'a' AS a`'s column statistics from
`SELECT 'a' AS a FROM tbl`.
1. If the foldable expression is null:
```scala
ColumnStat(Some(0), None, None, Some(rowCount), Some(size), Some(size),
None, 2)
```
2. If the foldable expression is not null:
```scala
ColumnStat(Some(1), Some(value), Some(value), Some(0), Some(size),
Some(size), None, 2)
```
### Why are the changes needed?
Improve column statistics.
### Does this PR introduce _any_ user-facing change?
No.
### How was this patch tested?
Unit test.
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