Add documentation about how to plan custom expressions

[alamb]

Is your feature request related to a problem or challenge?

By default DataFusion doesn't support many operators, for example ->

> set datafusion.sql_parser.dialect = postgres;
0 row(s) fetched.
Elapsed 0.001 seconds.

> select 'foo'->'bar';
This feature is not implemented: Unsupported SQL binary operator Arrow

However, you can use the ExprPlanner API to support htem

• There is a lower level API that lets you customize how expressions are planned, here: https://docs.rs/datafusion/latest/datafusion/logical_expr/planner/trait.ExprPlanner.html
• https://docs.rs/datafusion/latest/datafusion/execution/trait.FunctionRegistry.html#method.register_expr_planner

However, when @dadepo asked about this in #15264 it was not clear it was supported

After some digging I found an example in our tests:

datafusion/datafusion/core/tests/user_defined/expr_planner.rs

Lines 35 to 124 in 87eec43

	impl ExprPlanner for MyCustomPlanner {
	fn plan_binary_op(
	&self,
	expr: RawBinaryExpr,
	_schema: &DFSchema,
	) -> Result<PlannerResult<RawBinaryExpr>> {
	match &expr.op {
	BinaryOperator::Arrow => {
	Ok(PlannerResult::Planned(Expr::BinaryExpr(BinaryExpr {
	left: Box::new(expr.left.clone()),
	right: Box::new(expr.right.clone()),
	op: Operator::StringConcat,
	})))
	}
	BinaryOperator::LongArrow => {
	Ok(PlannerResult::Planned(Expr::BinaryExpr(BinaryExpr {
	left: Box::new(expr.left.clone()),
	right: Box::new(expr.right.clone()),
	op: Operator::Plus,
	})))
	}
	BinaryOperator::Question => {
	Ok(PlannerResult::Planned(Expr::Alias(Alias::new(
	Expr::Literal(ScalarValue::Boolean(Some(true))),
	None::<&str>,
	format!("{} ? {}", expr.left, expr.right),
	))))
	}
	_ => Ok(PlannerResult::Original(expr)),
	}
	}
	}
	async fn plan_and_collect(sql: &str) -> Result<Vec<RecordBatch>> {
	let config =
	SessionConfig::new().set_str("datafusion.sql_parser.dialect", "postgres");
	let mut ctx = SessionContext::new_with_config(config);
	ctx.register_expr_planner(Arc::new(MyCustomPlanner))?;
	ctx.sql(sql).await?.collect().await
	}
	#[tokio::test]
	async fn test_custom_operators_arrow() {
	let actual = plan_and_collect("select 'foo'->'bar';").await.unwrap();
	let expected = [
	"+----------------------------+",
	"| Utf8(\"foo\") || Utf8(\"bar\") |",
	"+----------------------------+",
	"| foobar |",
	"+----------------------------+",
	];
	assert_batches_eq!(&expected, &actual);
	}
	#[tokio::test]
	async fn test_custom_operators_long_arrow() {
	let actual = plan_and_collect("select 1->>2;").await.unwrap();
	let expected = [
	"+---------------------+",
	"| Int64(1) + Int64(2) |",
	"+---------------------+",
	"| 3 |",
	"+---------------------+",
	];
	assert_batches_eq!(&expected, &actual);
	}
	#[tokio::test]
	async fn test_question_select() {
	let actual = plan_and_collect("select a ? 2 from (select 1 as a);")
	.await
	.unwrap();
	let expected = [
	"+--------------+",
	"| a ? Int64(2) |",
	"+--------------+",
	"| true |",
	"+--------------+",
	];
	assert_batches_eq!(&expected, &actual);
	}
	#[tokio::test]
	async fn test_question_filter() {
	let actual = plan_and_collect("select a from (select 1 as a) where a ? 2;")
	.await
	.unwrap();
	let expected = ["+---+", "| a |", "+---+", "| 1 |", "+---+"];
	assert_batches_eq!(&expected, &actual);
	}

Describe the solution you'd like

I would like a secton in in the docs that has an example of how to do this

Perhaps in this page: https://datafusion.apache.org/library-user-guide/adding-udfs.html
Or maybe it needs its own page 🤔

Describe alternatives you've considered

No response

Additional context

No response

[Jiashu-Hu]

take

[alamb]

There might be an example in https://github.com/apache/datafusion/tree/main/datafusion-examples you could port to the docs

[Jiashu-Hu]

Thank you very much, I've done with this issue, will submit PR soon after test