### What changes were proposed in this pull request?
In order to avoid frequently changing the value of `spark.sql.adaptive.shuffle.maxNumPostShufflePartitions`, we usually set `spark.sql.adaptive.shuffle.maxNumPostShufflePartitions` much larger than `spark.sql.shuffle.partitions` after enabling adaptive execution, which causes some bucket map join lose efficacy and add more `ShuffleExchange`.
How to reproduce:
```scala
val bucketedTableName = "bucketed_table"
spark.range(10000).write.bucketBy(500, "id").sortBy("id").mode(org.apache.spark.sql.SaveMode.Overwrite).saveAsTable(bucketedTableName)
val bucketedTable = spark.table(bucketedTableName)
val df = spark.range(8)
spark.conf.set("spark.sql.autoBroadcastJoinThreshold", -1)
// Spark 2.4. spark.sql.adaptive.enabled=false
// We set spark.sql.shuffle.partitions <= 500 every time based on our data in this case.
spark.conf.set("spark.sql.shuffle.partitions", 500)
bucketedTable.join(df, "id").explain()
// Since 3.0. We enabled adaptive execution and set spark.sql.adaptive.shuffle.maxNumPostShufflePartitions to a larger values to fit more cases.
spark.conf.set("spark.sql.adaptive.enabled", true)
spark.conf.set("spark.sql.adaptive.shuffle.maxNumPostShufflePartitions", 1000)
bucketedTable.join(df, "id").explain()
```
```
scala> bucketedTable.join(df, "id").explain()
== Physical Plan ==
*(4) Project [id#5L]
+- *(4) SortMergeJoin [id#5L], [id#7L], Inner
:- *(1) Sort [id#5L ASC NULLS FIRST], false, 0
: +- *(1) Project [id#5L]
: +- *(1) Filter isnotnull(id#5L)
: +- *(1) ColumnarToRow
: +- FileScan parquet default.bucketed_table[id#5L] Batched: true, DataFilters: [isnotnull(id#5L)], Format: Parquet, Location: InMemoryFileIndex[file:/root/opensource/apache-spark/spark-3.0.0-SNAPSHOT-bin-3.2.0/spark-warehou..., PartitionFilters: [], PushedFilters: [IsNotNull(id)], ReadSchema: struct<id:bigint>, SelectedBucketsCount: 500 out of 500
+- *(3) Sort [id#7L ASC NULLS FIRST], false, 0
+- Exchange hashpartitioning(id#7L, 500), true, [id=#49]
+- *(2) Range (0, 8, step=1, splits=16)
```
vs
```
scala> bucketedTable.join(df, "id").explain()
== Physical Plan ==
AdaptiveSparkPlan(isFinalPlan=false)
+- Project [id#5L]
+- SortMergeJoin [id#5L], [id#7L], Inner
:- Sort [id#5L ASC NULLS FIRST], false, 0
: +- Exchange hashpartitioning(id#5L, 1000), true, [id=#93]
: +- Project [id#5L]
: +- Filter isnotnull(id#5L)
: +- FileScan parquet default.bucketed_table[id#5L] Batched: true, DataFilters: [isnotnull(id#5L)], Format: Parquet, Location: InMemoryFileIndex[file:/root/opensource/apache-spark/spark-3.0.0-SNAPSHOT-bin-3.2.0/spark-warehou..., PartitionFilters: [], PushedFilters: [IsNotNull(id)], ReadSchema: struct<id:bigint>, SelectedBucketsCount: 500 out of 500
+- Sort [id#7L ASC NULLS FIRST], false, 0
+- Exchange hashpartitioning(id#7L, 1000), true, [id=#92]
+- Range (0, 8, step=1, splits=16)
```
This PR makes read bucketed tables always obeys `spark.sql.shuffle.partitions` even enabling adaptive execution and set `spark.sql.adaptive.shuffle.maxNumPostShufflePartitions` to avoid add more `ShuffleExchange`.
### Why are the changes needed?
Do not degrade performance after enabling adaptive execution.
### Does this PR introduce any user-facing change?
No.
### How was this patch tested?
Unit test.
Closes#26409 from wangyum/SPARK-29655.
Authored-by: Yuming Wang <yumwang@ebay.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>
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