OlegWock opened a new pull request, #20844:
URL: https://github.com/apache/datafusion/pull/20844
## Which issue does this PR close?
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I figured it will be easier to submit PR right away as change doesn't look
controversial. I'm happy to create an issue and link it here if you'd prefer.
## Rationale for this change
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So short story is: in another project we'd like to use DataFusion's to
"build" operations on data and then submit resulting logical plan _somewhere_
to execute (likely not using DF to actually execute the query). Since those
plans never meant to be executed by DF we use `EmptyTable` as a base to bring
schema to DF without any actual data. `EmptyTable` scans not being serializable
prevents us from sending those plans to Python or over the wire. I believe this
change makes datafusion's LogicalPlan more portable and more usable outside of
datafusion's query executor.
Longer story: [VegaFusion](https://github.com/vega/vegafusion) does
server-side aggregation for Vega charts and is powered by DataFusion. We
recently added option to [use custom query/plan
executors](https://github.com/vega/vegafusion/pull/573), which allows user to
pass a schema (without data) to VegaFusion which will add all necessary
aggregations (but not execute them) and return a logical plan to user. They can
then outsource this plan to custom query executor (e.g. Spark). This is already
implemented and works. However, since VegaFusion is most commonly used through
Python bindings, we'd like to expose this API to Python too (and additionally
as part of gPRC API too) , which requires serializing built plans to protobuf.
Currently we use `EmptyTable` to bring schema without any data to DataFusion.
But since it can't be converted to protobuf, we're unable to expose this API.
We considered providing custom decoder/encoder, but that would work only for
gRPC case, but not Python a
s datafusion-python doesn't allow to provide custom decoder as far as I
understand.
## What changes are included in this PR?
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* Moved `EmptyTable` from `datafusion-core` into `datafusion-catalog` and
added backwards compatibility re-export (following pattern for other table
providers moved earlier)
* Added new `EmptyTableScanNode` to protobuf definitions
* Added encoding and decoding for new entity into `AsLogicalPlan for
LogicalPlanNode` implementation
## Are these changes tested?
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I added two roundtrip tests for the new node
## Are there any user-facing changes?
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`EmptyTable` can be imported from `datafusion-catalog` crate now, but old
crate (`datafusion-core`) still re-exports it, so this shouldn't be breaking
change
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P.S. Just to be explicit, code itself was written mostly by LLM (as I'm not
that proficient in Rust yet). I did review and test it though
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