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BaselineMetrics output_bytes converts every column to ArrayData for every batch

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@minhh-nguyen 已经在做这个了。

开始于 2026年10月7日。

  • #26105 来自 @minhh-nguyen —— 未关闭

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调研方向

从 datafusion/physical-expr-common/src/metrics/baseline.rs 开始，阅读 RecordOutput::record_output 和 get_record_batch_memory_size，了解 output_bytes 是如何记录的。将提议的 Array::get_array_memory_size 方法与现有的共享缓冲区计量方式进行比较。完成标准是：记录输出字节时不再将列转换为 ArrayData，同时 get_record_batch_memory_size 仍可用于内存计量。

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描述

Is your feature request related to a problem or challenge?

Every operator's BaselineMetrics records output_bytes for every output batch through RecordOutput::record_output (datafusion/physical-expr-common/src/metrics/baseline.rs), which calls get_record_batch_memory_size. That function converts each column, and each child of a nested column, to ArrayData and inserts every buffer's address into a HashSet, so that buffers shared between columns are counted once. Exact accounting matters for memory reservations; for a per-batch metric it is a cost every query pays, and it grows with the width and nesting of the schema rather than with the data.

Measured on main (c0e872f), release build, Apple M1 Max, one batch of Int32 struct columns:

batch get_record_batch_memory_size sum of Array::get_array_memory_size
35 struct columns x 20 fields, 100 rows 21.9 µs 1.6 µs
35 struct columns x 20 fields, 4,000 rows 15.1 µs 1.5 µs
5 struct columns x 5 fields, 4,000 rows 0.58 µs 0.05 µs

That is per operator per batch. We hit it running one wide plan repeatedly (a game's world, 35 struct columns, a few thousand rows, a few hundred operators, through Sail on DataFusion 55.1): in a sampled profile, BaselineMetrics::record_poll -> get_record_batch_memory_size -> StructArray::to_data was about 12% of executing the plan, and computing the metric as below took a run from 40 ms to 34 ms.

Describe the solution you'd like

Compute output_bytes without ArrayData, for example:

fn output_bytes(batch: &RecordBatch) -> usize {
    batch.columns().iter().map(|c| c.get_array_memory_size()).sum()
}

This can count a buffer shared between columns more than once (in the table above it reports 350,840 bytes against 280,000 for the 100-row batch, which shares nothing; the difference is the arrays' own structures), which seems acceptable for a metric; get_record_batch_memory_size would stay for memory accounting. Alternatively, output_bytes could be computed only when a metrics level that shows it is requested.

Describe alternatives you've considered

Making get_record_batch_memory_size itself cheaper for nested arrays (walking the arrays' buffers without building ArrayData) would help memory accounting too, but keeps the hash set per batch.

Additional context

Related: #26065 (struct literal comparisons through ArrayData in physical planning) and the planning-speed work in #19795.

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