[SPARK-23303][SQL] improve the explain result for data source v2 relations
## What changes were proposed in this pull request?
The proposed explain format:
**[streaming header] [RelationV2/ScanV2] [data source name] [output] [pushed filters] [options]**
**streaming header**: if it's a streaming relation, put a "Streaming" at the beginning.
**RelationV2/ScanV2**: if it's a logical plan, put a "RelationV2", else, put a "ScanV2"
**data source name**: the simple class name of the data source implementation
**output**: a string of the plan output attributes
**pushed filters**: a string of all the filters that have been pushed to this data source
**options**: all the options to create the data source reader.
The current explain result for data source v2 relation is unreadable:
```
== Parsed Logical Plan ==
'Filter ('i > 6)
+- AnalysisBarrier
+- Project [j#1]
+- DataSourceV2Relation [i#0, j#1], org.apache.spark.sql.sources.v2.AdvancedDataSourceV2$Reader3b415940
== Analyzed Logical Plan ==
j: int
Project [j#1]
+- Filter (i#0 > 6)
+- Project [j#1, i#0]
+- DataSourceV2Relation [i#0, j#1], org.apache.spark.sql.sources.v2.AdvancedDataSourceV2$Reader3b415940
== Optimized Logical Plan ==
Project [j#1]
+- Filter isnotnull(i#0)
+- DataSourceV2Relation [i#0, j#1], org.apache.spark.sql.sources.v2.AdvancedDataSourceV2$Reader3b415940
== Physical Plan ==
*(1) Project [j#1]
+- *(1) Filter isnotnull(i#0)
+- *(1) DataSourceV2Scan [i#0, j#1], org.apache.spark.sql.sources.v2.AdvancedDataSourceV2$Reader3b415940
```
after this PR
```
== Parsed Logical Plan ==
'Project [unresolvedalias('j, None)]
+- AnalysisBarrier
+- RelationV2 AdvancedDataSourceV2[i#0, j#1]
== Analyzed Logical Plan ==
j: int
Project [j#1]
+- RelationV2 AdvancedDataSourceV2[i#0, j#1]
== Optimized Logical Plan ==
RelationV2 AdvancedDataSourceV2[j#1]
== Physical Plan ==
*(1) ScanV2 AdvancedDataSourceV2[j#1]
```
-------
```
== Analyzed Logical Plan ==
i: int, j: int
Filter (i#88 > 3)
+- RelationV2 JavaAdvancedDataSourceV2[i#88, j#89]
== Optimized Logical Plan ==
Filter isnotnull(i#88)
+- RelationV2 JavaAdvancedDataSourceV2[i#88, j#89] (Pushed Filters: [GreaterThan(i,3)])
== Physical Plan ==
*(1) Filter isnotnull(i#88)
+- *(1) ScanV2 JavaAdvancedDataSourceV2[i#88, j#89] (Pushed Filters: [GreaterThan(i,3)])
```
an example for streaming query
```
== Parsed Logical Plan ==
Aggregate [value#6], [value#6, count(1) AS count(1)#11L]
+- SerializeFromObject [staticinvoke(class org.apache.spark.unsafe.types.UTF8String, StringType, fromString, input[0, java.lang.String, true], true, false) AS value#6]
+- MapElements <function1>, class java.lang.String, [StructField(value,StringType,true)], obj#5: java.lang.String
+- DeserializeToObject cast(value#25 as string).toString, obj#4: java.lang.String
+- Streaming RelationV2 MemoryStreamDataSource[value#25]
== Analyzed Logical Plan ==
value: string, count(1): bigint
Aggregate [value#6], [value#6, count(1) AS count(1)#11L]
+- SerializeFromObject [staticinvoke(class org.apache.spark.unsafe.types.UTF8String, StringType, fromString, input[0, java.lang.String, true], true, false) AS value#6]
+- MapElements <function1>, class java.lang.String, [StructField(value,StringType,true)], obj#5: java.lang.String
+- DeserializeToObject cast(value#25 as string).toString, obj#4: java.lang.String
+- Streaming RelationV2 MemoryStreamDataSource[value#25]
== Optimized Logical Plan ==
Aggregate [value#6], [value#6, count(1) AS count(1)#11L]
+- SerializeFromObject [staticinvoke(class org.apache.spark.unsafe.types.UTF8String, StringType, fromString, input[0, java.lang.String, true], true, false) AS value#6]
+- MapElements <function1>, class java.lang.String, [StructField(value,StringType,true)], obj#5: java.lang.String
+- DeserializeToObject value#25.toString, obj#4: java.lang.String
+- Streaming RelationV2 MemoryStreamDataSource[value#25]
== Physical Plan ==
*(4) HashAggregate(keys=[value#6], functions=[count(1)], output=[value#6, count(1)#11L])
+- StateStoreSave [value#6], state info [ checkpoint = *********(redacted)/cloud/dev/spark/target/tmp/temporary-549f264b-2531-4fcb-a52f-433c77347c12/state, runId = f84d9da9-2f8c-45c1-9ea1-70791be684de, opId = 0, ver = 0, numPartitions = 5], Complete, 0
+- *(3) HashAggregate(keys=[value#6], functions=[merge_count(1)], output=[value#6, count#16L])
+- StateStoreRestore [value#6], state info [ checkpoint = *********(redacted)/cloud/dev/spark/target/tmp/temporary-549f264b-2531-4fcb-a52f-433c77347c12/state, runId = f84d9da9-2f8c-45c1-9ea1-70791be684de, opId = 0, ver = 0, numPartitions = 5]
+- *(2) HashAggregate(keys=[value#6], functions=[merge_count(1)], output=[value#6, count#16L])
+- Exchange hashpartitioning(value#6, 5)
+- *(1) HashAggregate(keys=[value#6], functions=[partial_count(1)], output=[value#6, count#16L])
+- *(1) SerializeFromObject [staticinvoke(class org.apache.spark.unsafe.types.UTF8String, StringType, fromString, input[0, java.lang.String, true], true, false) AS value#6]
+- *(1) MapElements <function1>, obj#5: java.lang.String
+- *(1) DeserializeToObject value#25.toString, obj#4: java.lang.String
+- *(1) ScanV2 MemoryStreamDataSource[value#25]
```
## How was this patch tested?
N/A
Author: Wenchen Fan <wenchen@databricks.com>
Closes #20647 from cloud-fan/explain.
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