rustyconover opened a new issue, #401:
URL: https://github.com/apache/arrow-js/issues/401
`RecordBatch.numRows` returns 0 for zero-column batches deserialized from
IPC, even when the IPC message header specifies a non-zero length. Zero-column
RecordBatches are valid Arrow payloads and should preserve their row count
through serialization round-trips.
## Current behavior
1. An IPC stream contains a zero-column RecordBatch with `length: 100`
2. The IPC reader correctly passes `header.length` to `makeData`, and
`visitStruct` preserves it
3. But `new RecordBatch(schema, data)` calls `ensureSameLengthData` which
recomputes length as `chunks.reduce((max, col) => Math.max(max, col.length),
0)` — with zero children, this returns 0
## Expected behavior
`RecordBatch.numRows` should be 100 after deserializing a zero-column batch
with `length: 100`.
## Reproducer
```typescript
import { makeData, RecordBatch, Schema, Struct } from 'apache-arrow';
const schema = new Schema([]);
const data = makeData({ type: new Struct([]), length: 100, nullCount: 0,
children: [] });
const batch = new RecordBatch(schema, data);
console.log(batch.numRows); // 0 — expected 100
```
## Root cause
In `src/recordbatch.ts` line 84, the 2-arg constructor path calls:
```typescript
[this.schema, this.data] = ensureSameLengthData<T>(this.schema,
this.data.children as Data<T[keyof T]>[]);
```
`ensureSameLengthData` (line 323) defaults `maxLength` to
`chunks.reduce(...)`, which returns 0 for an empty array.
The 1-arg constructor path (line 102) already passes `length` explicitly and
does not have this bug.
## Fix
Pass `this.data.length` as the third argument to `ensureSameLengthData`,
which already accepts an optional `maxLength` parameter.
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