Github user wzhfy commented on a diff in the pull request:
    --- Diff: 
sql/core/src/test/scala/org/apache/spark/sql/StatisticsSuite.scala ---
    @@ -101,4 +101,47 @@ class StatisticsSuite extends QueryTest with 
SharedSQLContext {
           checkTableStats(tableName, expectedRowCount = Some(2))
    +  test("test column-level statistics for data source table created in 
InMemoryCatalog") {
    +    def checkColStats(colStats: BasicColStats, expectedColStats: 
BasicColStats): Unit = {
    +      assert(colStats.dataType == expectedColStats.dataType)
    +      assert(colStats.numNulls == expectedColStats.numNulls)
    +      assert(colStats.max == expectedColStats.max)
    +      assert(colStats.min == expectedColStats.min)
    +      if (expectedColStats.ndv.isDefined) {
    +        // ndv is an approximate value, so we just make sure we have the 
    +        assert(colStats.ndv.get >= 0)
    --- End diff --
    I think I may know what @hvanhovell meant. The standard deviation is the 
parameter we pass to HyperLogLogPlusPlus, so the code may look like this:
            // ndv is an approximate value, so we make sure we have the value, 
and it should be
            // within 3*SD's of the given rsd.
            assert(colStats.ndv.get >= 0)
            val rsd = spark.sessionState.conf.ndvMaxError
            val error = math.abs((colStats.ndv.get / 
expectedColStats.ndv.get.toDouble) - 1.0d)
            assert(error <= rsd * 3.0d, "Error should be within 3 std. errors.")
    What do you think?

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