[SPARK-20302][SQL] Short circuit cast when from and to types are structurally the same

## What changes were proposed in this pull request?
When we perform a cast expression and the from and to types are structurally the same (having the same structure but different field names), we should be able to skip the actual cast.

## How was this patch tested?
Added unit tests for the newly introduced functions.

Author: Reynold Xin <rxin@databricks.com>

Closes #17614 from rxin/SPARK-20302.

2 files changed, 45 insertions, 0 deletions

diff --git a/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/CastSuite.scala b/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/CastSuite.scala
index 8eccadbdd8..a7ffa884d2 100644
--- a/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/CastSuite.scala
+++ b/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/CastSuite.scala
@@ -813,4 +813,18 @@ class CastSuite extends SparkFunSuite with ExpressionEvalHelper {
     assert(cast(1.0.toFloat, DateType).checkInputDataTypes().isFailure)
     assert(cast(1.0, DateType).checkInputDataTypes().isFailure)
   }
+
+  test("SPARK-20302 cast with same structure") {
+    val from = new StructType()
+      .add("a", IntegerType)
+      .add("b", new StructType().add("b1", LongType))
+
+    val to = new StructType()
+      .add("a1", IntegerType)
+      .add("b1", new StructType().add("b11", LongType))
+
+    val input = Row(10, Row(12L))
+
+    checkEvaluation(cast(Literal.create(input, from), to), input)
+  }
 }
diff --git a/sql/catalyst/src/test/scala/org/apache/spark/sql/types/DataTypeSuite.scala b/sql/catalyst/src/test/scala/org/apache/spark/sql/types/DataTypeSuite.scala
index f078ef0133..c4635c8f12 100644
--- a/sql/catalyst/src/test/scala/org/apache/spark/sql/types/DataTypeSuite.scala
+++ b/sql/catalyst/src/test/scala/org/apache/spark/sql/types/DataTypeSuite.scala
@@ -411,4 +411,35 @@ class DataTypeSuite extends SparkFunSuite {
   checkCatalogString(ArrayType(createStruct(40)))
   checkCatalogString(MapType(IntegerType, StringType))
   checkCatalogString(MapType(IntegerType, createStruct(40)))
+
+  def checkEqualsStructurally(from: DataType, to: DataType, expected: Boolean): Unit = {
+    val testName = s"equalsStructurally: (from: $from, to: $to)"
+    test(testName) {
+      assert(DataType.equalsStructurally(from, to) === expected)
+    }
+  }
+
+  checkEqualsStructurally(BooleanType, BooleanType, true)
+  checkEqualsStructurally(IntegerType, IntegerType, true)
+  checkEqualsStructurally(IntegerType, LongType, false)
+  checkEqualsStructurally(ArrayType(IntegerType, true), ArrayType(IntegerType, true), true)
+  checkEqualsStructurally(ArrayType(IntegerType, true), ArrayType(IntegerType, false), false)
+
+  checkEqualsStructurally(
+    new StructType().add("f1", IntegerType),
+    new StructType().add("f2", IntegerType),
+    true)
+  checkEqualsStructurally(
+    new StructType().add("f1", IntegerType),
+    new StructType().add("f2", IntegerType, false),
+    false)
+
+  checkEqualsStructurally(
+    new StructType().add("f1", IntegerType).add("f", new StructType().add("f2", StringType)),
+    new StructType().add("f2", IntegerType).add("g", new StructType().add("f1", StringType)),
+    true)
+  checkEqualsStructurally(
+    new StructType().add("f1", IntegerType).add("f", new StructType().add("f2", StringType, false)),
+    new StructType().add("f2", IntegerType).add("g", new StructType().add("f1", StringType)),
+    false)
 }