## What changes were proposed in this pull request?
In DataSourceV2Strategy, it seems we eliminate the subqueries by mistake after normalizing filters.
We have a sql with a scalar subquery:
``` scala
val plan = spark.sql("select * from t2 where t2a > (select max(t1a) from t1)")
plan.explain(true)
```
And we get the log info of DataSourceV2Strategy:
```
Pushing operators to csv:examples/src/main/resources/t2.txt
Pushed Filters:
Post-Scan Filters: isnotnull(t2a#30)
Output: t2a#30, t2b#31
```
The `Post-Scan Filters` should contain the scalar subquery, but we eliminate it by mistake.
```
== Parsed Logical Plan ==
'Project [*]
+- 'Filter ('t2a > scalar-subquery#56 [])
: +- 'Project [unresolvedalias('max('t1a), None)]
: +- 'UnresolvedRelation `t1`
+- 'UnresolvedRelation `t2`
== Analyzed Logical Plan ==
t2a: string, t2b: string
Project [t2a#30, t2b#31]
+- Filter (t2a#30 > scalar-subquery#56 [])
: +- Aggregate [max(t1a#13) AS max(t1a)#63]
: +- SubqueryAlias `t1`
: +- RelationV2[t1a#13, t1b#14] csv:examples/src/main/resources/t1.txt
+- SubqueryAlias `t2`
+- RelationV2[t2a#30, t2b#31] csv:examples/src/main/resources/t2.txt
== Optimized Logical Plan ==
Filter (isnotnull(t2a#30) && (t2a#30 > scalar-subquery#56 []))
: +- Aggregate [max(t1a#13) AS max(t1a)#63]
: +- Project [t1a#13]
: +- RelationV2[t1a#13, t1b#14] csv:examples/src/main/resources/t1.txt
+- RelationV2[t2a#30, t2b#31] csv:examples/src/main/resources/t2.txt
== Physical Plan ==
*(1) Project [t2a#30, t2b#31]
+- *(1) Filter isnotnull(t2a#30)
+- *(1) BatchScan[t2a#30, t2b#31] class org.apache.spark.sql.execution.datasources.v2.csv.CSVScan
```
## How was this patch tested?
ut
Closes#24321 from francis0407/SPARK-27411.
Authored-by: francis0407 <hanmingcong123@hotmail.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>
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