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https://issues.apache.org/jira/browse/DRILL-7491?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17015587#comment-17015587
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ASF GitHub Bot commented on DRILL-7491:
---------------------------------------

paul-rogers commented on pull request #1955: DRILL-7491: Incorrect count() 
returned for complex types in parquet
URL: https://github.com/apache/drill/pull/1955#discussion_r366674972
 
 

 ##########
 File path: 
exec/java-exec/src/main/java/org/apache/drill/exec/physical/base/AbstractGroupScanWithMetadata.java
 ##########
 @@ -167,29 +167,46 @@ 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())) {
+      // When statistics for nested field exists, this is complex column which 
is present in table.
+      // But its nested fields statistics can't be used to extract 
tableRowCount for this column.
+      // So NO_COLUMN_STATS returned here to avoid problems described in 
DRILL-7491.
+      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;
 
 Review comment:
   The above is true, but non-interesting. The number of nulls is useful when 
estimating selectivity of things like `IS NULL` or `IS NOT NULL`. If we are 
interested in scans, then the null count is not helpful.
   
   Given the math here, is the meaning of this function, "return the number of 
rows with non-null values for this column"? If so, that is somewhat a backward 
request. More typical is to return the null count directly, or the percentage 
of nulls.
   
   A reason that this value is not as helpful as it could be is if we are using 
stats to estimate something like `WHERE x = 10 AND y IS NOT NULL`. First, we 
estimate table row count, which might have been adjusted via partitioning. Then 
we estimate the selectivity reduction due to `x = 10`. Then we estimate the 
further reduction due to `y IS NOT NULL`. We cannot do this math if all we have 
is a number *n* that tell us the number of non-null rows in the table; we'd 
have to work out the selectivity number ourselves from the table row count.
 
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> Incorrect count() returned for complex types in parquet
> -------------------------------------------------------
>
>                 Key: DRILL-7491
>                 URL: https://issues.apache.org/jira/browse/DRILL-7491
>             Project: Apache Drill
>          Issue Type: Bug
>          Components: Functions - Drill, Functions - Hive, Storage - Parquet
>    Affects Versions: 1.16.0, 1.17.0
>            Reporter: Igor Guzenko
>            Assignee: Igor Guzenko
>            Priority: Major
>              Labels: ready-to-commit
>             Fix For: 1.18.0
>
>         Attachments: hive_alltypes.parquet
>
>
> To reproduce use the attached file for {{hive_alltypes.parquet}} (this is 
> parquet file generated by Hive) and try count on columns *c13 - c15.*  For 
> example, 
> {code:sql}
> SELECT count(c13) FROM dfs.tmp.`hive_alltypes.parquet`
> {code}
> *Expected result:* {color:green}3 {color}
> *Actual result:* {color:red}0{color}



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