pepijnve commented on issue #17158:
URL: https://github.com/apache/datafusion/issues/17158#issuecomment-3265646839
@alamb I had a quick look at the current implementation that handles this. I
believe this case is now handled by `ConstEvaluator`. If I'm reading the code
correctly the implementation currently does not take associativity into
account. You can see the same thing happening with integer addition for
instance:
```
> explain select c1 + 1 + 2 + c2 from t;
+---------------+---------------------------------------------------------------------------------+
| plan_type | plan
|
+---------------+---------------------------------------------------------------------------------+
| logical_plan | Projection: t.c1 + Int64(1) + Int64(2) + t.c2
|
| | TableScan: t projection=[c1, c2]
|
| physical_plan | ProjectionExec: expr=[c1@0 + 1 + 2 + c2@1 as t.c1 +
Int64(1) + Int64(2) + t.c2] |
| | DataSourceExec: partitions=1, partition_sizes=[1]
|
| |
|
+---------------+---------------------------------------------------------------------------------+
```
The current implementation of `ConstEvaluator` sees
```
+
+
+
c1
1
2
c2
```
Since each `BinaryOp` tree contains a column reference the entire tree gets
marked as `can_evaluate: false`.
In our current query engine implementation and optimiser I handled this case
by modelling an associativity property (and commutativity for a similar
canonicalisation rewrite rule) at the binary operator/function level. When
encountering a tree of identical associative operators, the operands of the
tree are collected into a list and then each subsequent pair of operands is
tested for const evaluation.
So for `c1 + 1 + 2 + c2`
```
+
+
+
c1
1
2
c2
```
becomes
```
+
c1, 1, 2, c2
```
which simplifies to
```
+
c1, 6, c2
```
and then gets expanded back to
```
+
+
c1
6
c2
```
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