## What changes were proposed in this pull request?
Schema pruning doesn't work if nested column is used in where clause.
For example,
```
sql("select name.first from contacts where name.first = 'David'")
== Physical Plan ==
*(1) Project [name#19.first AS first#40]
+- *(1) Filter (isnotnull(name#19) && (name#19.first = David))
+- *(1) FileScan parquet [name#19] Batched: false, Format: Parquet, PartitionFilters: [],
PushedFilters: [IsNotNull(name)], ReadSchema: struct<name:struct<first:string,middle:string,last:string>>
```
In above query plan, the scan node reads the entire schema of `name` column.
This issue is reported by:
https://github.com/apache/spark/pull/21320#issuecomment-419290197
The cause is that we infer a root field from expression `IsNotNull(name)`. However, for such expression, we don't really use the nested fields of this root field, so we can ignore the unnecessary nested fields.
## How was this patch tested?
Unit tests.
Closes#22357 from viirya/SPARK-25363.
Authored-by: Liang-Chi Hsieh <viirya@gmail.com>
Signed-off-by: DB Tsai <d_tsai@apple.com>
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