okumin commented on code in PR #6244:
URL: https://github.com/apache/hive/pull/6244#discussion_r2767157356
##########
ql/src/java/org/apache/hadoop/hive/ql/udf/generic/GenericUDFWhen.java:
##########
@@ -144,6 +148,53 @@ static class WhenStatEstimator implements StatEstimator {
@Override
public Optional<ColStatistics> estimate(List<ColStatistics> argStats) {
+ return estimate(argStats, null);
+ }
+
+ @Override
+ public Optional<ColStatistics> estimate(List<ColStatistics> argStats,
List<ExprNodeDesc> argExprs) {
Review Comment:
I'm clarifying my understanding. Please let me know if I'm overlooking
something.
Let's assume the number of distinct values of `col_2` is 2, that of
`col_100` is 100, and that of `col_999` is 999.
The true NDV of the following expression is 3. The original implementation
returns 1, and this implementation returns 3.
```sql
CASE
WHEN category BETWEEN 0 AND 4 THEN 'CODE_00'
WHEN category BETWEEN 5 AND 9 THEN 'CODE_01'
ELSE 'CODE_ELSE'
END
```
That of this is 2. The original implementation returns 1, and this
implementation returns 2.
```sql
CASE
WHEN category BETWEEN 0 AND 4 THEN 'CODE_00'
WHEN category BETWEEN 5 AND 9 THEN 'CODE_01'
ELSE 'CODE_01'
END
```
That of this is 100, 101, or 102. The original implementation returns 100,
and this implementation returns 100.
```sql
CASE
WHEN category BETWEEN 0 AND 4 THEN 'CODE_00'
WHEN category BETWEEN 5 AND 9 THEN 'CODE_01'
ELSE col_100
END
```
That of this is 999 ~ 1100. The original implementation returns 999, and
this implementation returns 999.
```sql
CASE
WHEN category BETWEEN 0 AND 4 THEN 'CODE_00'
WHEN category BETWEEN 5 AND 9 THEN col_999
ELSE col_100
END
```
That of this is 6 ~ 8. The original implementation returns 2, and this
implementation returns 2.
```sql
CASE
WHEN category BETWEEN 0 AND 4 THEN 'CODE_00'
WHEN category BETWEEN 5 AND 9 THEN 'CODE_01'
WHEN category BETWEEN 10 AND 14 THEN 'CODE_02'
WHEN category BETWEEN 15 AND 19 THEN 'CODE_03'
WHEN category BETWEEN 20 AND 24 THEN 'CODE_04'
WHEN category BETWEEN 25 AND 29 THEN 'CODE_05'
ELSE col_2
END
```
I'd say the current patch does not introduce worse estimation in any case.
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