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
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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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