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Construction of user-defined table functions (UDTFs) should be async to allow for async schemas #10889
Description
Is your feature request related to a problem or challenge?
Accessing a TableProviders schema is a sync function call (see here). This means that the TableProvider must know its schema before construction.
DataFusion recently introduced TableFunctionImpl, which allows users to define a function to create a TableProvider. Unfortunately, this call method is sync, meaning that the user-defined table function must know its schema upfront in a non-blocking way. This isn't possible when implementing TableProviders, which might infer their schema async, like an HTTP connector that can connect to arbitrary sources with payloads only known once the response is streaming in.
Describe the solution you'd like
I propose we make the call method async to allow for async schemas and thus async table provider construction.
current code
use super::TableProvider;
use datafusion_common::Result;
use datafusion_expr::Expr;
use std::sync::Arc;
/// A trait for table function implementations
pub trait TableFunctionImpl: Sync + Send {
/// Create a table provider
fn call(&self, args: &[Expr]) -> Result<Arc<dyn TableProvider>>;
}
/// A table that uses a function to generate data
pub struct TableFunction {
/// Name of the table function
name: String,
/// Function implementation
fun: Arc<dyn TableFunctionImpl>,
}
impl TableFunction {
/// Create a new table function
pub fn new(name: String, fun: Arc<dyn TableFunctionImpl>) -> Self {
Self { name, fun }
}
/// Get the name of the table function
pub fn name(&self) -> &str {
&self.name
}
/// Get the function implementation and generate a table
pub fn create_table_provider(&self, args: &[Expr]) -> Result<Arc<dyn TableProvider>> {
self.fun.call(args)
}
}where the new trait would looks something like
/// A trait for table function implementations
pub trait TableFunctionImpl: Sync + Send {
/// Create a table provider
async fn call(&self, args: &[Expr]) -> Result<Arc<dyn TableProvider>>;
}Describe alternatives you've considered
I've worked around this by creating the table outside of data fusion, but I would prefer to use the table functions to achieve the same thing.
Additional context
No response
Activity
alamb commentedon Jun 14, 2024
I think this proposal sounds like a great idea to me, FWIW
Thank you for the suggestion @matthewgapp