paul-rogers commented on a change in pull request #1955: DRILL-7491: Incorrect 
count() returned for complex types in parquet
URL: https://github.com/apache/drill/pull/1955#discussion_r366676515
 
 

 ##########
 File path: 
exec/java-exec/src/main/java/org/apache/drill/exec/physical/base/AbstractGroupScanWithMetadata.java
 ##########
 @@ -167,29 +167,43 @@ public boolean isMatchAllMetadata() {
    */
   @Override
   public long getColumnValueCount(SchemaPath column) {
-    long tableRowCount, colNulls;
-    Long nulls;
     ColumnStatistics<?> columnStats = 
getTableMetadata().getColumnStatistics(column);
-    ColumnStatistics<?> nonInterestingColStats = null;
-    if (columnStats == null) {
-      nonInterestingColStats = 
getNonInterestingColumnsMetadata().getColumnStatistics(column);
-    }
+    ColumnStatistics<?> nonInterestingColStats = (columnStats == null)
+        ? getNonInterestingColumnsMetadata().getColumnStatistics(column) : 
null;
 
+    long tableRowCount;
     if (columnStats != null) {
       tableRowCount = 
TableStatisticsKind.ROW_COUNT.getValue(getTableMetadata());
     } else if (nonInterestingColStats != null) {
       tableRowCount = 
TableStatisticsKind.ROW_COUNT.getValue(getNonInterestingColumnsMetadata());
+      columnStats = nonInterestingColStats;
+    } else if (existsNestedStatsForColumn(column, getTableMetadata())
+        || existsNestedStatsForColumn(column, 
getNonInterestingColumnsMetadata())) {
+      return Statistic.NO_COLUMN_STATS;
     } else {
       return 0; // returns 0 if the column doesn't exist in the table.
     }
 
-    columnStats = columnStats != null ? columnStats : nonInterestingColStats;
-    nulls = ColumnStatisticsKind.NULLS_COUNT.getFrom(columnStats);
-    colNulls = nulls != null ? nulls : Statistic.NO_COLUMN_STATS;
+    Long nulls = ColumnStatisticsKind.NULLS_COUNT.getFrom(columnStats);
+    if (nulls == null || Statistic.NO_COLUMN_STATS == nulls || 
Statistic.NO_COLUMN_STATS == tableRowCount) {
+      return Statistic.NO_COLUMN_STATS;
+    } else {
+      return tableRowCount - nulls;
+    }
+  }
 
-    return Statistic.NO_COLUMN_STATS == tableRowCount
-            || Statistic.NO_COLUMN_STATS == colNulls
-            ? Statistic.NO_COLUMN_STATS : tableRowCount - colNulls;
+  /**
+   * For complex columns, stats may be present only for nested fields. For 
example, a column path is `a`,
+   * but stats present for `a`.`b`. So before making a decision that column is 
absent, the case needs
+   * to be tested.
+   *
+   * @param column   column path
+   * @param metadata metadata with column statistics
+   * @return whether stats exists for nested fields
+   */
+  private boolean existsNestedStatsForColumn(SchemaPath column, Metadata 
metadata) {
 
 Review comment:
   I wonder about the premise of this function. The Java doc suggests we have 
the path `a.b`. We want to know if `a` is an uninteresting column. Are we 
suggesting that we have metadata that says that `a.b` is uninteresting, but we 
have no metadata for `a` itself?
   
   Why would this be? Do we store columns in a non-structured way? That is, do 
we have a list of columns like 'a', 'b', 'c.d', 'e.f.g', 'c.h', and so on? 
Rather than `(a, b, c(d, g), e((g)))`?
   
   Further we seem to be assuming that we will gather stats for, say, `a.c`, 
but not `a.b`.
   
   The only place I can see that such complexity would make sense is to 
estimate cardinality for Flatten or Lateral Join. In such a case, might it make 
more sense to treat this recursively as a nested table? That is 'a` is a table 
and `a.b` and `a.c` are just `b` and `c` columns in the nested table.
   
   Given this, we *must* have a metadata entry for `a` (the map) in order to 
have metadata for any of its nested columns.
   
   So, again, I wonder if we've defined our metadata semantics as clearly as we 
might.

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