Update tests and add comments.

sql/core/src/test/scala/org/apache/spark/sql/SQLConfSuite.scala

@@ -32,11 +32,12 @@ class SQLConfSuite extends QueryTest with SharedSQLContext {
   }
 
   test("programmatic ways of basic setting and getting") {
+    // Set a conf first.
+    ctx.setConf(testKey, testVal)
+    // Clear the conf.
     ctx.conf.clear()
-    // The number of confs set after we call clear equals to the number of
-    // confs in TestSQLContext.overrideConfs (we always override these confs
-    // when we use TestSQLContext).
-    assert(ctx.getAllConfs.size === TestSQLContext.overrideConfs.size)
+    // After clear, only overrideConfs used by unit test should be in the SQLConf.
+    assert(ctx.getAllConfs === TestSQLContext.overrideConfs)
 
     ctx.setConf(testKey, testVal)
     assert(ctx.getConf(testKey) === testVal)

sql/core/src/test/scala/org/apache/spark/sql/SQLQuerySuite.scala

@@ -25,7 +25,7 @@ import org.apache.spark.sql.catalyst.DefaultParserDialect
 import org.apache.spark.sql.catalyst.errors.DialectException
 import org.apache.spark.sql.execution.aggregate
 import org.apache.spark.sql.functions._
-import org.apache.spark.sql.test.SharedSQLContext
+import org.apache.spark.sql.test.{TestSQLContext, SharedSQLContext}
 import org.apache.spark.sql.test.SQLTestData._
 import org.apache.spark.sql.types._
 
@@ -990,7 +990,17 @@ class SQLQuerySuite extends QueryTest with SharedSQLContext {
     val nonexistentKey = "nonexistent"
 
     // "set" itself returns all config variables currently specified in SQLConf.
-    assert(sql("SET").collect().size == 0)
+    assert(sql("SET").collect().size == TestSQLContext.overrideConfs.size)
+    sql("SET").collect().foreach { row =>
+      val key = row.getString(0)
+      val value = row.getString(1)
+      assert(
+        TestSQLContext.overrideConfs.contains(key),
+        s"$key should not exist in SQLConf.")
+      assert(
+        TestSQLContext.overrideConfs(key) === value,
+        s"The value of $key should be ${TestSQLContext.overrideConfs(key)} instead of $value.")
+    }
 
     // "set key=val"
     sql(s"SET $testKey=$testVal")

sql/core/src/test/scala/org/apache/spark/sql/test/TestSQLContext.scala

@@ -63,6 +63,10 @@ private[sql] class TestSQLContext(sc: SparkContext) extends SQLContext(sc) { sel
 }
 
 private[sql] object TestSQLContext {
+
+  /**
+   * A map used to store all confs that need to be overridden in sql/core unit tests.
+   */
   val overrideConfs: Map[String, String] =
     Map(
       // Fewer shuffle partitions to speed up testing.

sql/hive/src/main/scala/org/apache/spark/sql/hive/test/TestHive.scala

@@ -471,6 +471,10 @@ class TestHiveContext(sc: SparkContext) extends HiveContext(sc) {
 }
 
 private[hive] object TestHiveContext {
+
+  /**
+   * A map used to store all confs that need to be overridden in sql/hive unit tests.
+   */
   val overrideConfs: Map[String, String] =
     Map(
       // Fewer shuffle partitions to speed up testing.