peter-toth commented on code in PR #10868:
URL: https://github.com/apache/datafusion/pull/10868#discussion_r1635129770
##########
datafusion/optimizer/src/common_subexpr_eliminate.rs:
##########
@@ -515,14 +522,15 @@ fn build_common_expr_project_plan(
let mut fields_set = BTreeSet::new();
let mut project_exprs = common_exprs
.into_iter()
- .map(|(expr_id, expr)| {
+ .enumerate()
+ .map(|(index, (expr_id, expr))| {
Review Comment:
So I was thinking of a following example:
```
Projection: (a + b) as "c1", (a + b) as "c2", (a + 2) as "c3", (a + 1 + 1)
as "c4"
```
When this query gets optimized it might be the case that
`CommonSubexprEliminate` runs first and extracts `a + b` to `#0` so the plan
becomes:
```
Projection: #0 as "c1", #0 as "c2", (a + 2) as "c3", (a + 1 + 1) as "c4"
Projection: a + b as #0, a
```
But then some other rule can run and do e.g. contant folding so the plan
becomes:
```
Projection: #0 as "c1", #0 as "c2", (a + 2) as "c3", (a + 2) as "c4"
Projection: a + b as #0, a
```
And then in the 2nd optimizer cycle `CommonSubexprEliminate` runs again and
wants to extract `a + 2`, but as far as I see your PR uses the same 0 based
aliases again in the 2nd+ runs so the plan would become invalid:
```
Projection: #0 as "c1", #0 as "c2", #0 as "c3", #0 as "c4"
Projection: a + 2 as "#0", #0 // Here the 2nd rule run starts to use #0
alias again and the alias collides with the 1st rule run
Projection: a + b as "#0", a
```
I'm not fully familiar with all the optimizer rules in DataFusion, so there
might not be any "constant folding" one in:
https://github.com/apache/datafusion/blob/main/datafusion/optimizer/src/optimizer.rs#L239-L272,
but I hope you get the idea why the current change can cause issues.
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