akurmustafa commented on code in PR #13497:
URL: https://github.com/apache/datafusion/pull/13497#discussion_r1851353044
##########
datafusion/physical-expr/src/equivalence/properties.rs:
##########
@@ -1208,7 +1208,12 @@ fn is_constant_recurse(
return true;
}
let children = expr.children();
- !children.is_empty() && children.iter().all(|c|
is_constant_recurse(constants, c))
+ // When expression contains branch even if all children are constant
+ // final result may not be constant
+ let is_branched = expr.as_any().is::<CaseExpr>();
Review Comment:
After you comment, I thought about this. It turns out that my initial
approach is indeed wrong. `Datafusion` has 2 kinds of constant expressions. One
is `across_partitions`, other is `per partition`, indicate with
`across_partitions=false` flag. In following plan
```
02)--ProjectionExec: expr=[CASE WHEN name@0 = name1 THEN 0 WHEN name@0 =
name2 THEN 0.5 END as a]
03)----UnionExec
04)------ProjectionExec: expr=[name1 as name]
05)--------PlaceholderRowExec
06)------ProjectionExec: expr=[name2 as name]
07)--------PlaceholderRowExec
```
union has 2 children, each with constant `name` columns (since each source
has single row, each column is constant). `Union` transforms these constant
children as constant, but with `across_partitions=false` (which is correct).
Each partition still receives constant value from their perspective (By the way
`Union` doesn't merge any partition ). This is also correct for
`ProjectionExec`. At `ProjectionExec` column `name` is constant, where
`across_partitions=false`. However, in the current code, it is propagated as
`across_partitions=true`. This behaviour is the root cause of the issue. I
changed my implementation to fix this issue.
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