zanmato1984 commented on issue #43768:
URL: https://github.com/apache/arrow/issues/43768#issuecomment-2303644889
> Edit: I've try to write tests here and found it's actually bug-free:
>
> 1. `ExecBatch` will regarded as size == 1 when all input is scalar ( has
constant )
> 2. So, the size is always 1, this is also handled by
`PromoteExecSpanScalars`
>
> So, actually it's always 1 here
You are right in the context when solely compute kernels are involved. In
this case, you can assume that when the argument of `any` is scalar, then the
batch length must be `1`.
However this might not be the case in a more complex context, e.g. acero.
Here is a concrete test that reproduces the expected bug (explained at last):
```C++
TEST(ScalarAggregate, BuggyAny) {
std::shared_ptr<Schema> in_schema = schema({field("not_used", int32())});
std::vector<ExecBatch> batches{
ExecBatchFromJSON({int32()}, "[[42], [42], [42], [42]]")};
std::vector<Aggregate> aggregates = {
Aggregate("any",
std::make_shared<compute::ScalarAggregateOptions>(/*skip_nulls=*/false,
/*min_count=*/2),
FieldRef("literal_true"))};
Declaration plan = Declaration::Sequence(
{{"exec_batch_source", ExecBatchSourceNodeOptions(in_schema,
std::move(batches))},
{"project", ProjectNodeOptions({literal(true)}, {"literal_true"})},
{"aggregate", AggregateNodeOptions(aggregates)}});
ASSERT_OK_AND_ASSIGN(BatchesWithCommonSchema out_batches,
DeclarationToExecBatches(plan));
std::cout << out_batches.batches[0].values[0].ToString() << std::endl;
}
```
Output:
```
Scalar(null)
```
Explain: One source node with 1 batch of 4 rows (contents don't matter),
followed by a projection node which outputs literal `true` only (also 4 rows).
The tricky part is what this projection node emits: a batch of logically 4 rows
but of a single scalar column. When this batch is eventually ingested into the
subsequent aggregation node, which calls `any` on this scalar column with
`min_count` being `2`, boom.
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