Class Placeholder

datafusion/expr/src/expr.rs:1388–1393

/// or can be specified directly using `PREPARE` statements.
#[derive(Clone, PartialEq, Eq, PartialOrd, Hash, Debug)]
pub struct Placeholder {
    /// The identifier of the parameter, including the leading `$` (e.g, `"$1"` or `"$foo"`)
    pub id: String,
    /// The type the parameter will be filled in with
    pub field: Option<FieldRef>,
}

impl Placeholder {
    /// Create a new Placeholder expression