peter-toth commented on code in PR #10868:
URL: https://github.com/apache/datafusion/pull/10868#discussion_r1636669317
##########
datafusion/optimizer/src/common_subexpr_eliminate.rs:
##########
@@ -166,6 +166,15 @@ impl CommonSubexprEliminate {
) -> Result<(Vec<Vec<Expr>>, LogicalPlan)> {
let mut common_exprs = IndexMap::new();
+ input.schema().iter().for_each(|(qualifier, field)| {
Review Comment:
I don't think we should suppose that all extracted aliases remain in the
plan:
- Let's suppose that we have a plan like this after some CSE optimization:
```
...
Projection: #1 as c1, #1 as c2, #2 as c3, #2 as c4, (a + 2) as c5, (a + 1
+ 1) as c6
Projection: (a + b) as "#1", (a + c) as "#2", a
...
```
- Then some other optimization rule prunes "c1" and "c2" from the plan
because they turn out to be unnecessary:
```
...
Projection: #2 as c3, #2 as c4, (a + 2) as c5, (a + 1 + 1) as c6
Projection: (a + c) as "#2", a
...
```
- And then some other rule creates new CSE possibilities:
```
...
Projection: #2 as c3, #2 as c4, (a + 2) as c5, (a + 2) as c6
Projection: (a + c) as "#2", a
...
```
- CSE rule runs again but indexes here and at
`build_common_expr_project_plan()` are out of sync...
IMO the best thing we can do is to choose a unique aliases for a common
expressions in `CommonSubexprRewriter` when we found the expression and store
the alias in `common_exprs` together with the expression. In that case we don't
need to deal with index sync issues and don't get plans with unnecessary
aliases like here:
https://github.com/apache/datafusion/pull/10868/files#diff-351499880963d6a383c92e156e75019cd9ce33107724a9635853d7d4cd1898d0R1403
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