[SPARK-10859] [SQL] fix stats of StringType in columnar cache

The UTF8String may come from UnsafeRow, then underline buffer of it is not copied, so we should clone it in order to hold it in Stats.

cc yhuai

Author: Davies Liu <davies@databricks.com>

Closes #8929 from davies/pushdown_string.

sql/core/src/main/scala/org/apache/spark/sql/columnar/ColumnStats.scala

@@ -213,8 +213,8 @@ private[sql] class StringColumnStats extends ColumnStats {
     super.gatherStats(row, ordinal)
     if (!row.isNullAt(ordinal)) {
       val value = row.getUTF8String(ordinal)
-      if (upper == null || value.compareTo(upper) > 0) upper = value
+      if (upper == null || value.compareTo(upper) > 0) upper = value.clone()
-      if (lower == null || value.compareTo(lower) < 0) lower = value
+      if (lower == null || value.compareTo(lower) < 0) lower = value.clone()
       sizeInBytes += STRING.actualSize(row, ordinal)
     }
   }

sql/core/src/test/scala/org/apache/spark/sql/columnar/InMemoryColumnarQuerySuite.scala

@@ -212,4 +212,11 @@ class InMemoryColumnarQuerySuite extends QueryTest with SharedSQLContext {
     // Drop the cache.
     cached.unpersist()
   }
+
+  test("SPARK-10859: Predicates pushed to InMemoryColumnarTableScan are not evaluated correctly") {
+    val data = sqlContext.range(10).selectExpr("id", "cast(id as string) as s")
+    data.cache()
+    assert(data.count() === 10)
+    assert(data.filter($"s" === "3").count() === 1)
+  }
 }