Github user viirya commented on a diff in the pull request:

    https://github.com/apache/spark/pull/15090#discussion_r79331978
  
    --- 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 
value
    +        assert(colStats.ndv.get >= 0)
    --- End diff --
    
    Because the check here for estimated ndv value is not really related to the 
actual ndv value, we can't make sure if we really get the estimated ndv back. 
Especially we only use `ndv > 0` to check it. In an extreme case, we can 
replace the ndv expression with a literal(0) in the aggregation, the test would 
not check it out.


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