Function parse_expr

datafusion/functions-window/src/lead_lag.rs:343–363

When `lead`/`lag` is evaluated on a `NULL` expression we attempt to refine it by matching it with the type of the default value. For e.g. in `lead(NULL, 1, false)` the generic `ScalarValue::Null` is refined into `ScalarValue::Boolean(None)`. Only the type is refined, the expression value remains `NULL`. When the window function is evaluated with `NULL` expression this guarantees that the type ma

(
    input_exprs: &[Arc<dyn PhysicalExpr>],
    input_fields: &[FieldRef],
)

///
/// For more details see: <https://github.com/apache/datafusion/issues/12717>
fn parse_expr(
    input_exprs: &[Arc<dyn PhysicalExpr>],
    input_fields: &[FieldRef],
) -> Result<Arc<dyn PhysicalExpr>> {
    assert!(!input_exprs.is_empty());
    assert!(!input_fields.is_empty());

    let expr = Arc::clone(input_exprs.first().unwrap());
    let expr_field = input_fields.first().unwrap();

    // Handles the most common case where NULL is unexpected
    if !expr_field.data_type().is_null() {
        return Ok(expr);
    }

    let default_value = get_scalar_value_from_args(input_exprs, 2)?;
    default_value.map_or(Ok(expr), |value| {
        ScalarValue::try_from(&value.data_type())
            .map(|v| Arc::new(expressions::Literal::new(v)) as Arc<dyn PhysicalExpr>)
    })
}

static NULL_FIELD: LazyLock<FieldRef> =
    LazyLock::new(|| Field::new("value", DataType::Null, true).into());