[SPARK-50463][SQL] Fix ConstantColumnVector with Columnar to Row conversion

sql/catalyst/src/main/java/org/apache/spark/sql/vectorized/ColumnVector.java

@@ -69,14 +69,14 @@ public abstract class ColumnVector implements AutoCloseable {
   public abstract void close();
 
   /**
-   * Cleans up memory for this column vector if it's not writable. The column vector is not usable
-   * after this.
+   * Cleans up memory for this column vector if it's resources are freeable between batches.
+   * The column vector is not usable after this.
    *
-   * If this is a writable column vector, it is a no-op.
+   * If this is a writable column vector or constant column vector, it is a no-op.
    */
-  public void closeIfNotWritable() {
-    // By default, we just call close() for all column vectors. If a column vector is writable, it
-    // should override this method and do nothing.
+  public void closeIfFreeable() {
+    // By default, we just call close() for all column vectors. If a column vector is writable or
+    // constant, it should override this method and do nothing.
     close();
   }
 

sql/catalyst/src/main/java/org/apache/spark/sql/vectorized/ColumnarBatch.java

@@ -46,12 +46,12 @@ public void close() {
   }
 
   /**
-   * Called to close all the columns if they are not writable. This is used to clean up memory
-   * allocated during columnar processing.
+   * Called to close all the columns if their resources are freeable between batches.
+   * This is used to clean up memory allocated during columnar processing.
    */
-  public void closeIfNotWritable() {
+  public void closeIfFreeable() {
     for (ColumnVector c: columns) {
-      c.closeIfNotWritable();
+      c.closeIfFreeable();
     }
   }
 

sql/core/src/main/java/org/apache/spark/sql/execution/vectorized/ConstantColumnVector.java

@@ -77,6 +77,11 @@ public ConstantColumnVector(int numRows, DataType type) {
     }
   }
 
+  public void closeIfFreeable() {
+    // no-op: `ConstantColumnVector`s reuse the data backing its value across multiple batches and
+    // are freed at the end of execution in `close`.
+  }
+
   @Override
   public void close() {
     stringData = null;

sql/core/src/main/java/org/apache/spark/sql/execution/vectorized/WritableColumnVector.java

@@ -97,7 +97,7 @@ public void close() {
   }
 
   @Override
-  public void closeIfNotWritable() {
+  public void closeIfFreeable() {
     // no-op
   }
 

sql/core/src/main/scala/org/apache/spark/sql/execution/Columnar.scala

@@ -194,7 +194,7 @@ case class ColumnarToRowExec(child: SparkPlan) extends ColumnarToRowTransition w
        |    $shouldStop
        |  }
        |  $idx = $numRows;
-       |  $batch.closeIfNotWritable();
+       |  $batch.closeIfFreeable();
        |  $batch = null;
        |  $nextBatchFuncName();
        |}

sql/core/src/test/scala/org/apache/spark/sql/execution/datasources/parquet/ParquetQuerySuite.scala

@@ -473,6 +473,26 @@ abstract class ParquetQuerySuite extends QueryTest with ParquetTest with SharedS
     }
   }
 
+  test("SPARK-50463: Partition values can be read over multiple batches") {
+    withTempDir { dir =>
+      withSQLConf(SQLConf.PARQUET_VECTORIZED_READER_BATCH_SIZE.key -> "1") {
+        val path = dir.getAbsolutePath
+        spark.range(0, 5)
+          .selectExpr("concat(cast(id % 2 as string), 'a') as partCol", "id")
+          .write
+          .format("parquet")
+          .mode("overwrite")
+          .partitionBy("partCol").save(path)
+        val df = spark.read.format("parquet").load(path).selectExpr("partCol")
+        val expected = spark.range(0, 5)
+          .selectExpr("concat(cast(id % 2 as string), 'a') as partCol")
+          .collect()
+
+        checkAnswer(df, expected)
+      }
+    }
+  }
+
   test("SPARK-10301 requested schema clipping - same schema") {
     withTempPath { dir =>
       val path = dir.getCanonicalPath