kosiew commented on code in PR #22564:
URL: https://github.com/apache/datafusion/pull/22564#discussion_r3332260030
##########
datafusion/functions/src/math/log.rs:
##########
@@ -358,43 +386,83 @@ impl ScalarUDFImpl for LogFunc {
} else {
lit(ScalarValue::new_ten(&number_datatype)?)
};
+ let base_datatype = info.get_data_type(&base)?;
+
+ if is_zero_literal(&number, &number_datatype)?
+ || is_zero_literal(&base, &base_datatype)?
+ {
+ return Ok(ExprSimplifyResult::Original(original_log_args(
+ num_args, &base, &number,
+ )?));
+ }
+
+ let base_is_valid_literal = is_valid_log_base_literal(&base)?;
match number {
Expr::Literal(value, _)
- if value == ScalarValue::new_one(&number_datatype)? =>
+ if value == ScalarValue::new_one(&number_datatype)?
+ && base_is_valid_literal =>
{
Ok(ExprSimplifyResult::Simplified(lit(ScalarValue::new_zero(
&info.get_data_type(&base)?,
)?)))
}
Expr::ScalarFunction(ScalarFunction { func, mut args })
- if is_pow(&func) && args.len() == 2 && base == args[0] =>
+ if is_pow(&func)
+ && args.len() == 2
+ && base == args[0]
+ && base_is_valid_literal =>
Review Comment:
Thanks for tightening the rewrite conditions. I think there is still one
edge case here.
The `log(a, power(a, b)) => b` rewrite is still applied when `a` is a valid
literal base. That removes the runtime `log` evaluation, which also removes the
zero-value validation.
For example:
```sql
select log(10.0, power(10.0, column1))
from (values (-400.0), (2.0)) as t(column1);
```
With this rewrite, the result becomes `-400.0, 2.0`.
Without the rewrite, `power(10.0, -400.0)` underflows to `0.0`, so the first
row reaches `invoke_with_args` and raises `cannot take logarithm of zero`,
which matches the behavior of:
```sql
select log(10.0, power(10.0, -400.0));
```
Could we either avoid this rewrite unless the value can be proven non-zero,
or preserve the runtime zero-value validation when the value side is still an
expression? A regression test covering an exponent that underflows to zero
would also help lock in the expected behavior.
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