Add protobuf serialization/deserialization support for EmptyTable scans (#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](vega/vegafusion#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 as 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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updated before approving the PR.
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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

Author: OlegWock

datafusion/core/src/datasource/empty.rs datafusion/catalog/src/empty.rsdatafusion/core/src/datasource/empty.rs renamed to datafusion/catalog/src/empty.rs

@@ -21,15 +21,14 @@ use std::sync::Arc;
 
 use arrow::datatypes::*;
 use async_trait::async_trait;
-use datafusion_catalog::Session;
-use datafusion_common::project_schema;
-
-use crate::datasource::{TableProvider, TableType};
-use crate::error::Result;
-use crate::logical_expr::Expr;
+use datafusion_common::{Result, project_schema};
+use datafusion_expr::{Expr, TableType};
 use datafusion_physical_plan::ExecutionPlan;
 use datafusion_physical_plan::empty::EmptyExec;
 
+use crate::Session;
+use crate::TableProvider;
+
 /// An empty plan that is useful for testing and generating plans
 /// without mapping them to actual data.
 #[derive(Debug)]

datafusion/catalog/src/lib.rs

@@ -34,6 +34,7 @@
 
 pub mod cte_worktable;
 pub mod default_table_source;
+pub mod empty;
 pub mod information_schema;
 pub mod listing_schema;
 pub mod memory;

datafusion/core/src/datasource/mod.rs

@@ -20,7 +20,6 @@
 //! [`ListingTable`]: crate::datasource::listing::ListingTable
 
 pub mod dynamic_file;
-pub mod empty;
 pub mod file_format;
 pub mod listing;
 pub mod listing_table_factory;
@@ -39,6 +38,7 @@ pub use crate::catalog::TableProvider;
 pub use crate::logical_expr::TableType;
 pub use datafusion_catalog::cte_worktable;
 pub use datafusion_catalog::default_table_source;
+pub use datafusion_catalog::empty;
 pub use datafusion_catalog::memory;
 pub use datafusion_catalog::stream;
 pub use datafusion_catalog::view;

datafusion/proto/proto/datafusion.proto

@@ -62,6 +62,7 @@ message LogicalPlanNode {
     RecursiveQueryNode recursive_query = 31;
     CteWorkTableScanNode cte_work_table_scan = 32;
     DmlNode dml = 33;
+    EmptyTableScanNode empty_table_scan = 34;
   }
 }
 
@@ -1426,6 +1427,13 @@ message CteWorkTableScanNode {
     datafusion_common.Schema schema = 2;
 }
 
+message EmptyTableScanNode {
+    TableReference table_name = 1;
+    datafusion_common.Schema schema = 2;
+    ProjectionColumns projection = 3;
+    repeated LogicalExprNode filters = 4;
+}
+
 enum GenerateSeriesName {
   GS_GENERATE_SERIES = 0;
   GS_RANGE = 1;

datafusion/proto/src/generated/pbjson.rs

@@ -35,6 +35,7 @@ use crate::{
 use crate::protobuf::{ToProtoError, proto_error};
 use arrow::datatypes::{DataType, Field, Schema, SchemaBuilder, SchemaRef};
 use datafusion_catalog::cte_worktable::CteWorkTable;
+use datafusion_catalog::empty::EmptyTable;
 use datafusion_common::file_options::file_type::FileType;
 use datafusion_common::{
     Result, TableReference, ToDFSchema, assert_or_internal_err, context,
@@ -1065,6 +1066,35 @@ impl AsLogicalPlan for LogicalPlanNode {
                 )?
                 .build()
             }
+            LogicalPlanType::EmptyTableScan(scan) => {
+                let schema: Schema = convert_required!(scan.schema)?;
+                let schema = Arc::new(schema);
+                let mut projection = None;
+                if let Some(columns) = &scan.projection {
+                    let column_indices = columns
+                        .columns
+                        .iter()
+                        .map(|name| schema.index_of(name))
+                        .collect::<Result<Vec<usize>, _>>()?;
+                    projection = Some(column_indices);
+                }
+
+                let filters =
+                    from_proto::parse_exprs(&scan.filters, ctx, extension_codec)?;
+
+                let table_name =
+                    from_table_reference(scan.table_name.as_ref(), "EmptyTableScan")?;
+
+                let provider = Arc::new(EmptyTable::new(Arc::clone(&schema)));
+
+                LogicalPlanBuilder::scan_with_filters(
+                    table_name,
+                    provider_as_source(provider),
+                    projection,
+                    filters,
+                )?
+                .build()
+            }
             LogicalPlanType::Dml(dml_node) => {
                 Ok(LogicalPlan::Dml(datafusion_expr::DmlStatement::new(
                     from_table_reference(dml_node.table_name.as_ref(), "DML ")?,
@@ -1277,6 +1307,19 @@ impl AsLogicalPlan for LogicalPlanNode {
                             },
                         )),
                     })
+                } else if provider.downcast_ref::<EmptyTable>().is_some() {
+                    let schema: protobuf::Schema = schema.as_ref().try_into()?;
+
+                    Ok(LogicalPlanNode {
+                        logical_plan_type: Some(LogicalPlanType::EmptyTableScan(
+                            protobuf::EmptyTableScanNode {
+                                table_name: Some(table_name.clone().into()),
+                                schema: Some(schema),
+                                projection,
+                                filters,
+                            },
+                        )),
+                    })
                 } else {
                     let schema: protobuf::Schema = schema.as_ref().try_into()?;
                     let mut bytes = vec![];