amansinha100 commented on a change in pull request #1699: DRILL-7108: Improve
selectivity estimates for (NOT)LIKE, NOT_EQUALS, IS NOT NULL predicates
URL: https://github.com/apache/drill/pull/1699#discussion_r266491303
##########
File path:
exec/java-exec/src/main/java/org/apache/drill/exec/planner/common/DrillStatsTable.java
##########
@@ -134,6 +135,33 @@ public Double getRowCount() {
return rowCount > 0 ? rowCount : null;
}
+ /**
+ * Get the approximate number of distinct values of given column. If stats
are not present for the
+ * given column, a null is returned.
+ *
+ * Note: returned data may not be accurate. Accuracy depends on whether the
table data has changed
+ * after the stats are computed.
+ *
+ * @param col - column for which approximate count distinct is desired
+ * @return approximate count distinct of the column, if available. NULL
otherwise.
+ */
+ public Double getNNRowCount(String col) {
+ // Stats might not have materialized because of errors.
+ if (!materialized) {
+ return null;
+ }
+ final String upperCol = col.toUpperCase();
+ Long nnRowCntCol = nnRowCount.get(upperCol);
+ if (nnRowCntCol == null) {
+ nnRowCntCol =
nnRowCount.get(SchemaPath.getSimplePath(upperCol).toString());
+ }
+ // Ndv estimation techniques like HLL may over-estimate, hence cap it at
rowCount
Review comment:
Update for NN row count.
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