[Spark] Fall back to zordering when clustering on a single column (#3109

)

## Description
Fall back to zorder when clustering on a single column, because hilbert
clustering doesn't support 1 column.

Resolves #3087 

## How was this patch tested?
New unit test.

spark/src/main/scala/org/apache/spark/sql/delta/skipping/MultiDimClustering.scala

@@ -50,6 +50,7 @@ object MultiDimClustering {
       curve: String): DataFrame = {
     assert(colNames.nonEmpty, "Cannot cluster by zero columns!")
     val clusteringImpl = curve match {
+      case "hilbert" if colNames.size == 1 => ZOrderClustering
       case "hilbert" => HilbertClustering
       case "zorder" => ZOrderClustering
       case unknownCurve =>

spark/src/test/scala/org/apache/spark/sql/delta/clustering/ClusteredTableClusteringSuite.scala

@@ -79,4 +79,28 @@ class ClusteredTableClusteringSuite extends SparkFunSuite
       }
     }
   }
+
+  test("cluster by 1 column") {
+    withSQLConf(SQLConf.MAX_RECORDS_PER_FILE.key -> "2") {
+      withClusteredTable(
+        table = table,
+        schema = "col1 int, col2 int",
+        clusterBy = "col1") {
+        addFiles(table, numFiles = 4)
+        val files0 = getFiles(table)
+        assert(files0.size === 4)
+        assertNotClustered(files0)
+
+        // Optimize should cluster the data into two 2 files since MAX_RECORDS_PER_FILE is 2.
+        runOptimize(table) { metrics =>
+          assert(metrics.numFilesRemoved == 4)
+          assert(metrics.numFilesAdded == 2)
+        }
+
+        val files1 = getFiles(table)
+        assert(files1.size == 2)
+        assertClustered(files1)
+      }
+    }
+  }
 }