haohuaijin opened a new pull request, #23082:
URL: https://github.com/apache/datafusion/pull/23082
## Which issue does this PR close?
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- Closes #.
## Rationale for this change
`HashJoinExec` and `NestedLoopJoinExec` carry `projection:
Option<Vec<usize>>` but the proto field is `repeated uint32`. Proto3 can't tell
`None` from `Some(vec![])`, and the decoder treats both as `None`.
`Some(vec![])` is reachable in real plans — `try_embed_projection` produces it
for `SELECT count(1) … JOIN …` (#20191) — and after a round-trip the join
silently switches from "emit zero columns" to "emit all columns".
`FilterExec` has a workaround for the same limitation; these two execs were
missed.
## What changes are included in this PR?
Encode `Some(vec![])` as the single-element sentinel `[u32::MAX]` (never a
valid column index); recognise it on decode. Everything else goes through
unchanged.
```rust
// encode
projection: match exec.projection.as_ref() {
None => Vec::new(),
Some(v) if v.is_empty() => vec![u32::MAX],
Some(v) => v.iter().map(|x| *x as u32).collect(),
},
// decode
let projection = match hashjoin.projection.as_slice() {
[] => None,
[u32::MAX] => Some(Vec::new()),
indices => Some(indices.iter().map(|i| *i as usize).collect()),
};
```
Applied symmetrically to `HashJoinExec` and `NestedLoopJoinExec`.
## Are these changes tested?
yes, add roundtrip test case
## Are there any user-facing changes?
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