thomasrebele commented on code in PR #6293:
URL: https://github.com/apache/hive/pull/6293#discussion_r2919486102
##########
ql/src/java/org/apache/hadoop/hive/ql/optimizer/calcite/stats/FilterSelectivityEstimator.java:
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@@ -184,91 +189,383 @@ public Double visitCall(RexCall call) {
return selectivity;
}
+ /**
+ * Return whether the expression is a removable cast based on stats and type
bounds.
+ *
+ * <p>
+ * In Hive, if a value cannot be represented by the cast, the result of the
cast is NULL,
+ * and therefore cannot fulfill the predicate. So the possible range of the
values
+ * is limited by the range of possible values of the type.
+ * </p>
Review Comment:
The case of `TINYINT` to `SMALLINT` is covered by " if a value cannot be
represented by the cast", or rather by its negation. As all values of `TINYINT`
can be represented by the cast to `SMALLINT`, we can see it as a case of the
first category, where all non-representable values are cast to NULL. It's just
that with `TINYINT` to `SMALLINT` there are no non-representable values, so the
condition is trivially true. I'll try to improve the wording a bit.
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